Abstract
Intensive exploration and evaluation of cytomorphological diversity has been carried out on 380 species of 127 genera belonging to 28 families of sub-class Polypetalae of flowering plants from Kashmir (Jammu and Kashmir) and Kangra and Sirmaur districts (Himachal Pradesh) of the Western Himalayas. The cytological investigations of these species over a period of three years revealed new and varied chromosome numbers for 100 species globally and 50 species in India, making a substantial addition to the knowledge of the genera to which these species belong. To obtain a comprehensive cytological picture of each of these genera, chromosomal data have been updated by compiling the literature on previous chromosomal numbers and supplementing it from the present studies. The final form is now ready to show the status both at global and Indian levels for various parameters like total number of taxonomically known species, number of cytologically determined species along with intraspecifically added number of cytological taxa, presently inferred basic numbers, level and frequency of polyploids, and information on number of species per genus carrying inter- and intraspecific euploid and aneuploid variability at the genus level. Of the total 127 genera, 39 genera have 75% or more cytologically worked out species. Addition of cytotypes in many cytologically known species has resulted in an enhanced number of chromosomal races/cytological taxa over such species, in the majority of genera, justifying the ever growing need to make population based intensive studies of any plant species. The data show that monobasic and dibasic genera are less common than tribasic and polybasic ones. Genera with x = 8 are most common, followed by x = 7 and x = 6. Of 127 genera, 47 genera exhibit polyploidy of up to 25%; 22 genera have 26–50%; 15 genera have 51–70%; and 26 genera have 76–100% polyploidy, while 17 genera lack polyploidy altogether. Interspecific and/or intraspecific euploid cytotypes such as diploids plus polyploids or with “polyploid series” are present in most of the 107 genera. Intraspecific aneuploid chromosome numbers are also shown by 100 genera. Since these genera belong to different families, so no generalization can be made at family level. However, at genus level chromosomal observations show the active role of various evolutionary processes responsible for chromosomal diversity in the majority of these genera distributed in the Western Himalayas of India.
Introduction
As a part of our program to explore and evaluate genetic diversity of Indian angiosperms in general, and polypetalous plants in particular (Bir and Kumari Citation1979, Citation1981a, Citation1981b; Kumari and Bir Citation1985, Citation1987, Citation1989, Citation1990), an incentive was received to carry out population-based cytological studies on members of this group from selected phytogeographical areas of the Western Himalayas with altitude ranging from 400 to 4500 m. Plant material has been collected from higher altitude localities of Kashmir and from Sirmaur and Kangra districts (Himachal Pradesh) for the first time. More than four years of continuous effort to collect the wild germplasm and study detailed population-based male meiosis in 380 polypetalous species has provided vital cytological information, especially regarding the variability of intraspecific chromosome numbers. To understand this fully, it has been decided to discuss first the overall chromosome numbers of the genera to which these species belong. Therefore, the analysis of chromosome numbers of 127 genera belonging to 28 families of Polypetalae is presented in this paper on the basis of cumulative worldwide information available from the previous literature along with additions made from our present investigations (Rani et al. Citation2010a, Citation2010b; Jeelani, Rani, et al. Citation2010, Citation2011a, Citation2011b, Citation2013; Rani, Kumari and Gupta Citation2011a, 2011b; Jeelani, Kumari and Gupta Citation2011a, 2011b, 2012a, 2012b; Kumar, Kumari, et al. Citation2011, 2012; Kumar, Jeelani, Rani, Gupta, et al. Citation2011, 2013a, 2013b; Kumar, Jeelani, Rani, Kumari, et al. Citation2011, 2013; Rani, Kumar, Jeelani et al. Citation2011, 2012, 2013; Rani, Gupta and Kumari Citation2012; Rani, Kumari, Gupta et al. Citation2013).
Number of taxonomically known species
It is important to assess the frequency of cytologically determined species of each genus studied before analyzing their chromosomal data. To work out this parameter it is essential to have up-to-date data of taxonomically known species at global and Indian levels for each genus. A perusal of floristic accounts given in different Floras provides different figures for the number of taxonomic species in the genera, and an effort has been made to include the latest version both at global and Indian levels. Hence, the data given in column II of Table is based on taxonomic records available from different floras and research papers. For world records, the information is obtained from different floras including efloras of China (http://www.efloras.org/flora_page.aspx?flora_id=2) and Pakistan (http://www.efloras.org/browse.aspx?flora id=5) as well as research papers, whereas data for India are from Santapau and Henry (Citation1973), Aswal and Mehrotra (Citation1994), Sharma and Balakrishnan (Citation1993), Sharma and Sanjappa (Citation1993), Sharma et al. (Citation1993) and Hajra et al. (Citation1997) and many research papers.
Table 1. Cytological overview of polypetalous genera investigated from parts of the western Himalayas, on the basis of complete information including previously reported chromosome numbers as well as those from the current study.
Number of cytologically known species
The chromosome number data sources include chromosomal atlases (Darlington and Wylie Citation1955; Fedorov 1974; Kumar and Subramaniam Citation1986; Khatoon and Ali Citation1993), chromosome number indexes (Ornduff Citation1968, 1969; Moore 1970–1977; Goldblatt 1981–1988; Goldblatt and Johnson Citation1990–2003), IAPT/IOPB and SOCGI chromosome reports, as well as more recent findings of chromosome numbers by us. This variety of sources highlights that chromosome number lists have multiplied tremendously since the time of Darlington and Wylie (Citation1955).
From the literature it is clear that during the past few decades the number of chromosomally known species of flowering plants has rapidly increased. Likewise, there have been major contributions to the cytology of polypetalous plants. Important contributors from outside India include Zhou et al. (Citation2002), Lihová et al. (Citation2003), Ghaffari (Citation2004), Yan-Jun et al. (Citation2006), Wang et al. (Citation2008), Sheng et al. (Citation2010), Gholipour and Sheidai (Citation2010), Gömürgen et al. (Citation2011), Ranjbar et al. (Citation2012), Chung et al. (Citation2013), and de Resende et al. (Citation2013). Major contributions over the past few decades from India include Sharma and Sarkar (Citation1967–1968), Sharma (Citation1970), Roy and Sharma (Citation1971), Chatterjee and Sharma (Citation1972), Sanjappa (Citation1979), Hore (Citation1971, Citation1980), Panigrahi and Purohit (Citation1984), Subramanian (Citation1985), Govindarajan and Subramanian (Citation1986) and Vaidya and Joshi (Citation2003). Some of the recent contributors for the Western Himalayan polypetalous plants in particular include Pimenov et al. (Citation2006), Kumar and Singhal (Citation2008, Citation2011, Citation2013), Singhal and Kumar (Citation2008), Gupta et al. (Citation2009), Singhal and Kaur (Citation2009), Singhal et al. (Citation2009, Citation2010, Citation2011), Kaur et al. (Citation2010), and Kumar et al. (Citation2010).
The number and frequency of cytologically determined species, as shown in column III of Table , reflects the attention received from cytologists at the Indian level in the backdrop of the worldwide status of these genera. Considering a 75–100% frequency as an arbitrary threshold level of cytologically known species, it is seen that certain genera have achieved more attention at global (39 genera) and Indian levels (54 genera). Frequencies of 50–75% can be taken as a moderate level, and are represented by 32 and 30 genera at global and Indian levels, respectively. Genera with fewer than 50% cytologically reported species can be taken as a lower frequency level, seeking more attention in 56 and 43 genera at global and Indian levels, respectively. So whatsoever has been cytologically accomplished for these genera till now is being analyzed here to estimate the role of various evolutionary processes in this limited stock of genera belonging to Polypetalae, met with in the Western Himalayas (India).
Number of cytological taxa/cytotypes
In column number VII of Table , the number of cytotypes/chromosomal races is given against the total number of cytologically known species of the genus (column III), pointing out the increasing number of cytological taxa for each genus. Further, segregation of the number of the total cytological taxa of any genus leads to an insight into the range and frequency of increasing order of 2n chromosome numbers with number of cytotypes carrying particular chromosome numbers given in parenthesis. This information is a collective measure of inter- and intraspecific variability of the genera at global and Indian levels. It is further witnessed that most of the genera have predominantly 2n chromosome number variability, except for seven genera with a single cytotype each and lacking variation: Bergenia, Coronopus, Dalbergia, Ferula, Myricaria, Parochetus and Selenium at the global level. It is interesting to note that there are 26 other genera with chromosome number variability in species from outside India, however, in India these are represented by a single cytotype for each genus (see Table ).
Variability of chromosome numbers
From the literature, nine genera have the lowest 2n chromosome numbers, of less than 10; however these reports have not been confirmed and hence may be considered exceptional. These include Viola modesta 2n = 4 (Erben Citation1996), Arabidopsis thaliana 2n = 6 (Titova Citation1935), Impatiens leschenaultia 2n = 6 (Zinoveva-Stahevith and Grant Citation1982), Hypericum undulatum 2n = 8 (Guillén et al.Citation1997), Impatiens latifolia 2n = 8 (Rao et al. Citation1986; Ayyangar et al. Citation1987), Indigofera richardsiae 2n = 8 (Frahm Leliveled 1966), Pelargonium elongatum 2n = 8 (Gibby and Westfold Citation1986), Sanicula rupiflora 2n = 8 (Dobeš et al. Citation1997), Trifolium longipes 2n = 8 (Darlington and Wylie Citation1955), Viola dirimliensis 2n = 8 (Parolly and Eren Citation2006); and Lathyrus pratensis, 2n = 9 (Dobeš et al. Citation1997).
Taking lower chromosome numbers as established common diploid numbers for calculating the primary basic numbers, 87 genera are recognized with lowest chromosome number, as 2n = 10 (eight genera), 2n = 11 (one genus, Sesbania, with an odd and exceptional number, hence not to be counted for calculating basic numbers), 2n = 12 (19 genera), 2n = 14 (25 genera), 2n = 16 (21 genera), and 2n = 18 (13 genera).The remaining 31 genera with similar details and lowest chromosome numbers have 2n = 20 (seven genera), 2n = 22 (seven genera), 2n = 24 (five genera), 2n = 26 (four genera), 2n = 28 (one genus), 2n = 32 (three genera), 2n = 34 (one genus) 2n = 36 (one genus) and 2n = 40 (two genera).
Some genera have a wide range of chromosome numbers. In such genera chromosome numbers reached as 2n = 100 or more than this previously. In certain genera some species have higher chromosome numbers, e.g. Acacia (A. heburclada 2n = 208), Arenaria (A. ciliate 2n = 100, 120, 160, 200); Geranium (G. anemonifolium 2n = 68, 128; G. regelii 2n = 52,128), Meconopsis (M. grandis 2n = 164), Palargonium (P. roseum 2n = 154), Ranunculus (R. glabrifolius 2n = 144), Sedum (S. farinosum 2n = c.384; S. ebracteatum 2n = 40 + 1B, 80 + 0 – 2B, 160, 180, 200, 210; S. rupestre 2n = 56, 122, 140, 168, S. sexangulare 2n = 74, 111, 148, 185), Silene (S. ciliata 2n = 24, 25, 26, 36, 48, 120, 149–165, 155, 192, 228, 264, 312); Stellaria (S. palustris 2n = 198) and Thalictrum (T. dasycarpumi 2n = 168). Detailed explanation for these high numbers is not always available in the literature, although some may represent natural polyploids, and others may be the result of tissue culture material studies or artificially produced polyploids, etc.
Basic numbers of genera
Before discussing the ploidy level, it is essential to know the basic chromosome numbers of the taxa. An inference of an accurate basic numbers is little cumbersome in those genera which are marked with large amount of chromosome numbers variability. From the cytological literature, generally the basic numbers are based on gametic numbers of the species with the lowest 2n chromosome numbers in the genus. However, in some of the genera, high chromosome numbers are presumed to be multiples of lower numbers which do not actually exist (Stebbins Citation1958), and thus considered for taking their gametic numbers to be accepted as basic numbers. To help further, sometimes other criteria are also used, e.g. the number of nucleolar chromosomes in a complement (Gates Citation1942), or the number of chromosomes with secondary constrictions per complement (cf. Sharma Citation1976) or secondary associations of the chromosomes during meiosis-I (Darlington and Moffett Citation1930; Lawrence Citation1931; Moffett Citation1931), but each such method has its own limitation. Raven (Citation1975) has suggested that a prerequisite for calculating the original basic number of any group is a wide knowledge of its phylogeny. In line with a proposal given by Grant (Citation1982a, Citation1982b), that sufficient data pertaining to chromosome numbers is a prerequisite for calculating the basic chromosome numbers of a genus and consideration has to be given to the maximum number of species showing the particular gametic number and due importance is to be given to those chromosome numbers on which intraspecific euploid series are formed.
Regarding the genera under consideration, the basic numbers were suggested a long time ago and are clearly shown in the chromosome atlas of Darlington and Wylie (Citation1955), except for Pleurosperum, Vicatia, Alysicarpus, Flemingia, Uraria and Abelmoschus, which were probably not determined at that time. Fernandes and Franca (Citation1975) cited basic numbers of the genera pertaining to different families studied from Mozambique and in 1978 gave similar information regarding legumes from Portugal. Later on, Grant (Citation1982a) published a monographic work, “Periodicities in the chromosome numbers of the angiosperms”, on the basis of extensive information available from chromosome number compilations appearing up to 1974. He suggested the basic numbers for polyploid series in monocotyledonous and dicotyledonous genera by taking into account a particular basic number and the genus along with its related family in a clear form. In this way, on the basis of 7952 cytologically determined species of dicotyledons alone and gametic chromosome numbers given for these taxa ranging from n = 2 to n = 250, he evaluated data separately for herbaceous and woody species in the paper, and the picture is very clear for each genus to know which basic number(s) makes euploid series. From India, Kumari and Bir (Citation1987) compiled the chromosome numbers of all legumes and then worked out the basic numbers of all the 337 genera that were cytologically known by that time globally. Since then, there have been a large number of studies adding to the information on chromosome numbers of these flowering plants. Recently, Garbari et al. (Citation2012) presented a concise history of chromosome numbers of the Italian flora from 1925 to the present. Such studies, however, provide knowledge on chromosome numbers mostly of selected groups of plants from particular areas only and hence result in scattered information. So, it is necessary to revise the basic numbers in all the polypetalous genera worked out at present in the light of currently updated chromosomal data before accurately assessing the role of evolutionary processes such as polyploidy in these genera.
Having a fresh look on the range of 2n chromosome numbers and number of species or cytological taxa of 127 genera belonging to 28 families sharing these numbers, it is realized that basic numbers of all these genera cannot be calculated or described uniformly by using any single criterion. One thing is clear from the literature that generally basic chromosome numbers have been deciphered mathematically and the basic numbers making euploid series, however, are definitely taken as established numbers. At the same time, there are chromosome numbers in certain genera which do not fit in this measure, including diploids and singled out polyploids, and in many cases these are coupled with aneuploid variations. In such cases the basic chromosome numbers of any plant group need to be more accurately inferred after adding factors such as interrelationships with allied taxa, especially at intraspecific levels through population-based study covering a wide range of altitudes and habitats. The criteria adopted here are explained below.
Regarding primary basic number(s), it is important to see the frequency of cytological taxa based on particular basic numbers to deduce these, which is decided on the basis of maximum species/cytological taxa supporting such number(s). Out of these, one basic number with maximum depiction is regarded as common basic number as shown by underlying such numbers in column X of Table . However, in some of the genera, more than one basic number finds very high representation making the situation complex and has to be accepted as another common basic number. There is no doubt that in certain genera, the high frequency of more successful cytological taxa are available with basic numbers which are secondarily derived, along with relatively less frequent taxa marked with primary basic numbers.
There are nine genera as discussed earlier and marked with very low 2n chromosome numbers, even lower than commonly accepted numbers as 2n = 10. Further, it is noted from the literature that such cytotypes are represented mostly by single or a few taxa. An overview of chromosome numbers variations in these, otherwise cytologically well-studied genera give only one option to consider these extremely low chromosome numbers possibly either to be of haploid plants or some experimentally handled materials or some unique plants. Hence, the status of low chromosome numbers in such cases is to be taken with caution and their “half numbers taken as basic numbers” are shown with question mark in column X of Table . In fact, for consideration of level of ploidy, these “2n numbers” are straightway considered as “equivalent to basic numbers”. Some of the basic numbers arbitrarily calculated from stray/sporadic associated chromosome numbers, in certain species are to be taken with caution and casually regarded as doubtful as shown in parenthesis in column X of Table .
Basic numbers and categorization of genera
1. As recorded in most genera (86 of a total of 127 genera), the most authentic way remains the same as previously adopted as a general and popular method by various scientists i.e., the gametic number(s) of the species with lowest 2n chromosome number or a few conjunctive lower numbers of the euploid series to be taken as basic number(s). These are further subcategorized (a) typical ones as all those fitting strictly to this basic rule and (b) those which also carry some other aneuploid chromosome numbers in certain species, existing mostly as associated numbers along with regular numbers, and thus, ignored for inferring basic numbers. These are given below.
1a. Monobasic. In all, 16 genera are strictly monobasic, i.e. the species existing mainly as diploids or also having polyploids, but based on single basic number. This information is available in the literature, and here the same basic numbers are just confirmed but on the basis of revised data. These genera are Agrimonia (x = 14), Albizia (x = 13), Berberis (x = 14), Bergenia (x = 17), Coronopus (x = 16), Dalbergia (x = 10), Ferula (x = 11), Geum (x = 7), Grewia (x = 9), Murraya (x = 9), Myricaria (x = 12), Parochetus (x = 8), Sanicula (x = 8), Selinum (x = 11), Sibbaldia (x = 7) and Urena (x = 7).
1b. Monobasic (some associated 2n aneuploid reports ignored). These are 15 genera including Aeschynomene (x = 10), Argemone (x = 7), Barbarea (x = 8), Circaea (x = 11), Clematis (x = 8), Delphinium (x = 8), Desmodium (x = 11), Fragaria (x = 7), Lychnis (x = 12), Melilotus (x = 8), Oenothera (x = 7), Oxytropis (x = 8), Potentilla (x = 7), Rubus (x = 7), and Vicatia (x = 11).
1c. Dibasic. In all, 13 genera belong here: Alysicarpus (x = 8, 10), Atylosia (8, 11), Boenninghausenia (x = 9, 10), Capsella (x = 6, 8), Casealpinia (x = 11, 12), Descurainia (x = 7,10), Leucaena (x = 13, 14), Nigella (x = 6, 7), Podophyllum (x = 6, 8), Prosopis (x = 13, 14), Rhynchosia (x = 11, 12), Tribulus (x = 6, 10) and Zornia (x = 10, 11).
1d. Dibasic (some associated 2n aneuploid reports ignored). These eight genera include Filipendula (x = 7, 8), Hedysarum (x = 7, 8), Lathyrus (x = 6, 7), Nasturtium (x = 8, 11), Oenanthe (x = 10, 11), Ranunculus (x = 7, 8), Rosa (x = 6, 7) and Spiraea (x = 8, 9).
1e. Tribasic. These are eight genera including Caragana (x = 8, 9, 10), Chaerophyllum (x = 6, 7, 11), Crotalaria (x = 7, 8, 10), Flemingia (x = 9, 10, 11), Indigofera (x = 6, 7, 8), Lavatera (7, 10, 11), Momordica (x = 8, 11, 14) and Uraria (x = 8, 10, 11).
1f. Tribasic (some associated 2n aneuploid reports ignored). These 12 genera include Aquilegia (x = 7, 8, 9), Bupleurum (x = 6, 7, 8), Corchorus (x = 7, 8, 9), Dolichos (x = 10, 11, 12), Fumaria (x = 6, 7, 8), Lespedeza (x = 9, 10, 11), Lotus (x = 5, 6, 7), Medicago (x = 7, 8, 9), Sesbania (x = 6, 7, 8), Silene (x = 9, 10, 12), Thalictrum (x = 6, 7, 8) and Vicia (x = 5, 6, 7).
1g. Polybasic. These five genera are Argyrolobium (x = 13, 14, 15, 16), Daucus (x = 8, 9, 10, 11), Pimpinella (x = 8, 9, 10, 11), Pueraria (x = 10, 11, 12, 16) and Tephrosia (x = 11, 12, 13, 16).
1h. Polybasic (some associated 2n aneuploid reports ignored). These are 11 genera including Aconitum (x = 8, 10, 12, 13, 17), Anemone (x = 5, 7, 8, 9, 12), Arabis (x = 5, 6, 7, 8, 9), Bauhinia (x = 8, 12, 13, 14), Corydalis (x = 5, 6, 7, 8), Papaver (x = 6, 7, 9, 11), Scandix (x = 7, 8, 9, 10, 11), Sida (x = 6, 7, 8, 9), Sium (x = 6, 9, 10, 11), Stellaria (x = 9, 10, 11, 12, 13, 14) and Trifolium (x = 5, 6, 7, 8).
2. In the case of 12 genera with relatively fewer cytologically determined species per genus, some new basic numbers are added here to the already established basic numbers, as follows.
2a. In nine genera, proposed basic numbers are those which either (i) make euploid series or (ii) have been reported independently in one or more than one species as listed in Table .
2b. The basic numbers proposed here are half of some of the 2n dysploid numbers existing independently as successful cytotypes in some species. (Some odd chromosome numbers occurring only associated with established numbers in the same species are ignored for calculating basic numbers.) Two examples of such genera include Acacia (x = 13, 14, 19, 20) with only 18.11% species being determined with the established basic number x = 13, where proposed basic numbers are x = 14, 19, 20 (ignored 2n chromosome numbers are 39, 44) and Thlaspi (x = 7, 9, 12, 13) with common basic number x = 7, where proposed basic numbers are x = 9, 12, 13 (ignored chromosome number is 2n = 40).
2c. In case of one genus Epilobium, having x = 9, 10, 12, 13, 16, the proposed basic numbers are x = 10, 12, 13, 16, showing a common basic number of x = 9. Earlier Raven (Citation1988) proposed an ancestral number x = 18 for this genus. However, due to the availability of 2n = 18 in four species (Table ), x = 9 has to be taken as basic number on the basis of the lowest gametic number which also makes euploid series. Further, due to occurrence of polyploid series as 2n = 20, 30 as well as 2n = 26 present independently in three species, x = 10 and x = 13, respectively are to be retained. Basic numbers x = 12 for 2n = 24 and x = 16 for 2n = 32 are doubtful and need to be taken with caution because these chromosome numbers have never been confirmed again for any species.
3. There are 14 genera exhibiting a dysploid series of 2n chromosome numbers, thus with a polybasic nature in the form of dysploid basic numbers, which exist independently. Further, variable trends are there and may be noted as ascending or descending or either both ascending and descending series in relation to most common basic chromosome numbers of these genera. The examples are Arenaria x = 7, 8, 9, 10, 11, 12, 13 (common basic numbers x = 10, 11); Arabidopsis x = 5, 6, 7, 8, 9, 10, 11, 13 (common basic number x = 8; a few higher numbers might be the result of hybridization followed by diploidization of the lower numbers); Astragalus x = 6, 7, 8, 11, 12, 13 (common basic number x = 8); Caltha x = 8, 10, 12, 14 (common basic number x = 8); Cardamine x = 6, 7, 8, 9, 10, 17 (common basic number x = 8); Geranium x = 9, 10, 11, 12, 13, 14, 15, 16, 17, 23 (common basic number x = 14); Hypericum x = 7, 8, 9, 10, 12, 19 (common basic numbers x = 8, 9); Impatiens x = 5, 6, 7, 8, 9, 10 (common basic numbers x = 7, 8, 9 in agreement with the proposal of Song et al. [Citation2003] that frequent basic numbers are x = 7, 8, 9 and 10); Lupinus x = 7, 9, 12, 16, 17, 19, 20, 21, 22, 25, 26 (common basic number x = 12). Oxalis x = 5, 6, 7, 8, 9 (common basic number x = 7); Pelargonium x = 7, 8, 9, 10, 11, 12, 15 (common basic number x = 11); Saxifraga x = 5, 6, 7, 8, 9, 10, 13 (common basic number x = 8, supporting the earlier observation by Kumar Jeelani, Rani, Gupta, et al. [2011]); Sedum x = 5, 6, 7, 8, 9, 10, 11, 13 (common basic numbers x = 7, 8, 9, supporting the earlier postulations of Ehrendorfer [1963] that dysploid changes of the basic chromosome numbers in Sedum are probably due to chromosome fusion or fission rather than to aneuploidy, and also supporting t’Hart [1991] that cytological variations in Sedum are due to dysploid changes at the diploid as well as at the polyploid levels); and Viola x = 5, 6, 7, 8, 9, 11, 13, 17 (common basic numbers x = 5, 9, 13).
4. There are six genera showing relatively higher basic chromosome numbers. It is sometimes supposed that higher basic numbers arise from lower numbers of presumed diploids which do not exist, hence these higher basic numbers are regarded to be paleobasic in nature, arising through hybridization coupled with diploidization of lower numbers as proposed earlier by Grant (Citation1982b) for Erythrina and Hebe (x = 21), Fraxinus and Osmanthus (x = 23), Doronicum (x = 30), and Tilia (x = 41). At present, such genera include: Malva x = 12, 18, 20, 21 (these might have arisen from x = 6, 9, 10, 11); Malvastrum x = 12, 15, 16, 17, 18, 21, 22 (the common number is x = 12 and paleoploids are also coupled with dysploid ascending numbers); Prinsepia x = 14, 16 (possibly paleobasic because x = 7, 8 is a common base number in the many allied genera of the family); Pyrus x = 17, 21 (possibly paleobasic because x = 7 is a common base number in the many allied genera of the family); Abelmoschus x = 18, 20 (these might have arisen from x = 9, 10); and Gypsophila x = 6, 10, 12, 13, 15, 17 (this shows a normal basic number of x = 6 plus other discontinuous ascending basic numbers with the most prevalent numbers being x = 15 and 17).
5. There are three genera that show the presence of taxa with diploid numbers for which basic numbers are taken as their half numbers, but that also carry taxa with polyploid numbers which are not the multiples of the same basic numbers; these can be explained only by presuming supplementary basic numbers on the basis of nonexistent hypothetical diploid taxa. In Abutilon with x = 7, 8, 9, diploid and polyploid taxa suggest x = 7 and x = 8 as basic numbers, but a cytotype with 2n = 36 can be explained as tetraploid only on the basis of a presumed additional basic number of x = 9. In Heracleum with x = 10, 11, the count of 2n = 40 reported in three species as individual numbers or associated with 2n = 20, can be explained as tetraploid only on the basis of presumed basic number as x = 10. In Rorippa with x = 6, 8, a chromosome number of 2n = 28 is reported independently in four different species; this can be explained as tetraploid only on the basis of a presumed basic number of x = 7.
6. There are four genera with miscellaneous details as follows.
6a. Cerastium shows x = 9, 10, 17, 19, where more common basic numbers are x = 9, 10, and less common basic numbers are x = 17, 19. x = 17 is calculated on the basis that 2n = 34 is found in a significant number of species. Similarly, 2n = 38, is also noted in 18 species; hence, to be based on x = 19. Further, these higher numbers are likely to be paleobasic, arising through hybridization of the lower numbers 9 and 10. Boşcaiu et al. (1999), however, suggested x = 18 to be the main and secondary evolved basic number for this genus, giving a clear statement that there is no Cerastium species with 2n = 18. While making the statement, perhaps they did not take into account 2n = 18 already reported in C. lethospermifolium (Krogulevich Citation1971), which was later confirmed as one of the cytotypes in C. semidecandrum, i.e. 2n = 18, 36, 37 (Dmitrieva Citation2000).
6b. For Trigonella with x = 7, 8, 9, 11, there is no doubt that x = 8 is the most common basic number, but it is proposed that the number x = 7 also supports the series with 2n = 21 and 2n = 28, both cytotypes being present in two different species. Another basic number (x = 9) is inferred from two different species exhibiting 2n = 18. Since seven cytotypes have 2n = 44, these are supposed to be paleoploids based on x = 11. However, some odd chromosome numbers (2n = 17, 31) associated with regular euploid chromosome reports seem to be an outcome of frequent hybridization and cultivation in a few species. For Meconopsis with x = 7, 11, on the basis of previous information alone, variations in the chromosome numbers are shown ranging from 2n = 22 to 164. Interestingly, the present study reports for the first time the diploid cytotype of M. latifolia from Kashmir with 2n = 14. This settles the debate of whether x = 7 or x = 14 is the primary basic number of the genus, in favor of x = 7. Some ambiguous chromosome numbers, e.g. 2n = 74, 76, 82, and even higher numbers such as c.118 and 164, are found to be associated with higher regular chromosomes numbers, hence such numbers can be ignored for deciding the basic chromosome numbers. However, earlier, x = 7 and x = 8 (Ernst Citation1965; Ratter Citation1968) as well as x = 7 and x = 11 (Darlington and Wylie Citation1955) were suggested to be the most common basic numbers in the genus.
6c. For Alchemilla there have been problems in determining the exact chromosome numbers, the basic numbers and karyograms (Izmailow Citation1982). The basic numbers of Alchemilla are suggested to be x = 8, 10, 17 at present. Otherwise, x = 7 being also the basic number of the Rosoideae, has been accepted well earlier for this genus (see Gentcheff and Gustafsson Citation1940). Löve and Löve (Citation1975) and Raven (Citation1975), however, assumed a primary basic number of x = 8, which has been accepted by most authors since then. Here, x = 8 is also seen to be the most common basic number. The basic number of x = 10 seems to be coming from lower numbers as evident from making series only in polyploids. The other chromosome number x = 17 is decided, since it makes euploid series in nine species.
Polyploidy
Incidence
Polyploidy in angiosperms has been studied for almost a century now, dating back to the work of De Vries (see Gates Citation1909). The importance of polyploidy in evolution and speciation of plants has been emphasized by Kuwada (Citation1915), Müntzing (Citation1936), Darlington (Citation1937), Löve and Löve (Citation1949), Stebbins (Citation1950, Citation1971, Citation1985), Wendel and Doyle (Citation2005), Cui et al. (Citation2006), Otto (Citation2007), Wood et al. (Citation2009) and Meng et al. (Citation2012). Polyploidy is an important process in the evolutionary history of plants and has a profound impact on biodiversity dynamics and ecosystem functioning (Wendel Citation2000; Ainouche and Jenczewski Citation2010). Polyploidy and its occurrence within one species is a common phenomenon among plant groups (Soltis and Soltis Citation1993; Wendel Citation2000; Soltis et al. Citation2004; Hodálová et al. Citation2007; Ojiewo et al. Citation2007). Following the work of Stebbins (Citation1940, Citation1950) in particular, polyploidy became a major focus of biosystematic research. Manton (Citation1932) and Stebbins (Citation1950) have said that polyploidy may induce diversity of form and speciation, but has no significance in the origin of new major taxonomic groups. Further, polyploidy is supposed to protect plants against immediate deleterious effects of most gene mutations (Aase Citation1935) and thereby it allows greater polymorphism and thus polyploids attain greater adaptability (Stebbins Citation1950). As a result, plant scientists have long recognized that polyploid lineages may have complex relationships with each other and their diploid ancestors, making application of species concepts problematic (reviewed in Rieseberg and Willis Citation2007; Soltis et al. Citation2007). As polyploidy is so important it has been thoroughly investigated in the genera studied herein. In Table the number and frequency (based on total number of chromosomally reported species) of polyploid species of each genus are shown in column V and the level of euploids is shown in column VI. From the analysis of this data on world-wide basis, it is categorized further, to have deeper insight of this parameter as presented in Table providing information on all the 127 genera.’ It is noted that there are 17 genera lacking polyploidy; 47 with up to 25% polyploidy; 22 with 26–50% polyploidy; 15 with 51–75% polyploidy; and 26 with 76–100% polyploidy. Thus there are more genera with up to 25% polyploidy. The most common polyploidy level shared by almost all the genera is tetraploid, except for in genus Berberis (at hexaploid level) and another unique example of genus Murraya (at 12x level). The highest polyploidy level is quite varied, exhibited at different levels in different genera as 4x, 6x, 8x, 10x, 11x, 12x, 14x, 16x, 18x, 26x, 28x and 48x (Table ). The lowest level, 4x, belongs to genera Momordica and Podophyllum, and the highest level, 48x, belongs to genus Sedum (also see Table ).
Table 3. The information pertaining to 127 polypetalous genera studied at present with details of polyploidy.
Polyploidy and habit correlation
According to Stebbins (Citation1971) and de Wet (Citation1980), the origin and success of polyploidy quite often depends upon habit–habitat relationship and breeding system. According to Stebbins (Citation1938, Citation1950, Citation1971), “higher percentages of polyploidy within a modern genus are found in perennial herbs and lowest in annuals. The figures for woody plants are intermediate but approach more nearly those for annual than for perennial herbs”. de Wet (Citation1980) explained this by suggesting that a high rate of polyploidy in perennials could be their characteristic habit, providing repeated chances to sort out desirable combinations in the newly found polyploids so as to compete better with available habitat. Wright (Citation1976), however, rejected the concept of polyploidy–habit correlation. The genera studied here conform to the growth habit shown for each genus in Table and are categorized as: annual herbs with all five genera being polyploids; of 33 perennial herb genera 26 are polyploids; of 39 annual, biennial and perennial herb genera 37 are polyploids; of 22 genera with both herb and shrub habit 17 are polyploids; all eight genera including herbs, shrubs and trees are polyploids; of seven genera represented only by shrubs four are polyploids; and of 13 woody genera (shrubs and trees only), 12 are polyploids. Regarding the frequency of genera with different habits represented by different symbols from A–G as shown in the footnote of Table , it is inferred that overall perennials have a higher level of polyploidy and genera with a woody habit show frequency of polyploidy which lies in between the genera marked with annual and perennial herbs. These observations conform to those of Stebbins (Citation1971).
Euploid variations
Euploid variations are prevalent in most of the genera, as evident from the presence of diploids along with polyploids or only polyploid complexes in 890 species belonging to 107 genera of 26 families globally, and 99 species belonging to 49 genera of 22 families in India, as shown in column VIII of Table . The list of such species cannot be provided here, therefore only the number of species with more than one intraspecific euploid cytotype with their basic numbers are mentioned for each genus. In fact this column represents the story of more successful base numbers responsible for producing cytotypes of euploid series within any species belonging to such genera at India level in the background of the global picture.
Aneuploid variations
Aneuploid differentiation at the diploid level contributes greatly to species diversification in a genus (Wang et al. Citation2013). According to Stebbins (Citation1950, Citation1971, Citation1974), aneuploidy is the result of series of unequal translocations. Jones (Citation1978) has attributed aneuploidy to centric fusions. In Grant’s (Citation1982b) aneuploid–polyploid hypothesis, at lower levels of chromosome numbers paleopolyploidy becomes a less likely factor and basic aneuploidy becomes more important. Levin (Citation2002) also discussed the role of aneuploidy in relation to a shift in life history and asexual mode of reproduction. Aneuploidy is often correlated to the asexual mode of reproduction (apomixis). Nassar (Citation2003) studied cytological and embryological details of the apomictic clones of Manihot esculenta and correlated its occurrence to the aneuploid nature of the clones. According to De La Casa-Esperon and Sapienza (Citation2003) and Bean et al. (Citation2004) aneuploidy might be alleviated by the epigenetic silencing of unpaired chromosomes. Meiotic irregularities and a high rate of non-disjunction may also lead to production of aneuploids. According to Bandel (Citation1974), aneuploid variations form a series in which the gametic numbers of related species form consecutive series. The data on the existence of aneuploidy in 127 genera under consideration at present is given in column IX of Table . A total of 746 species of 104 genera globally and 118 species of 47 genera from India show chromosome numbers in the form of irregular multiplication of base numbers, and may be diploid or polyploid or both diploid and polyploid. These aneuploid variations at intraspecific level given here for a specific number of species per genus shows the frequency of such variants found in India in the light of global data, thereby supplementing the genetic diversity revealed through euploid variability.
Conclusion
For the first time, chromosome numbers of polypetalous plants from cytologically explored area of the Western Himalayas have been compiled. The complete chromosomal database is prepared not only on the basis of the literature but also substantiated from present detailed population-based meiotic studies from a vast area of the higher altitudinal Himalayas of Kashmir and Himachal Pradesh. The complete variability in chromosome numbers at Indian and global level revels the genetic diversity at intraspecific level within each genus, along with interspecific variability. The base numbers for all the genera have been reconsidered in the light of updated chromosomal data and presented in the most acceptable form. An exact assessment of the role of polyploidy and aneuploidy has been made available to ascertain their role in evolution of species belonging to these genera. An effort has been made to present the complete knowledge regarding chromosome number information for these 127 genera, for future use by researchers in different taxonomic treatments.
Acknowledgments
The authors are grateful to the University Grants Commission, New Delhi, for the award of Dr. D.S. Kothari Post-Doctoral Fellowship to Dr. Syed Mudassir Jeelani and Rajiv Gandhi National Fellowship to Dr. Sanjeev Kumar. We are also obliged to Department of Science and Technology, New Delhi, for the honour of Young Scientist Fellowship to Dr. Savita Rani. Thanks are also due to the Head, Department of Botany, Punjabi University, Patiala, for the library facilities.
Related Research Data
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