Phylogenetic Significance of the Pores in Urediospores

SUMMARY

1.	The data presented in this paper clearly indicate that the scattered arrangement of several pores is the primitive condition for urediospores, since such spores are: (1) commoner in the Melampsoraceae than in the Pucciniaceae, (2) not specialized in shape, (3) usually provided with colorless or nearly colorless walls, (4) commonly produced in uredia having peridia or paraphyses, (5) significantly associated, in Puccinia-Uromyces, with heteroecism, aecidioid aecia and primitive teliospores, and (6) tend to predominate on the more primitive groups of hosts of the genera Puccinia and Uromyces. The equatorial arrangement of few pores is correlated with the opposed character in each of the above categories, which indicates that it is an advanced character, phylogenetically.
2.	There is no evidence indicating that these changes have been detrimental, and the decrease in the number of pores and the increase in pigmentation are believed to increase the longevity of the spores. It is also suggested that the loss of uredial peridia and paraphyses, believed by previous writers to be protective structures, has been compensated by the development of urediospores with pigmented walls and fewer pores.
3.	The present tendency in Puccinia-Uromyces appears to be toward parallel changes which would ultimately yield autoecious species with uredinoid aecia, urediospores with few equatorial pores, and teliospores with uniformly thick, sculptured walls and fragile pedicels. The aecia and uredia have been eliminated with increasing frequency as this theoretically ultimate morphology has been approached.
4.	Judged on the basis of the arrangement of urediospore-pores the Pucciniastreae appears to be a closely related group of genera which has probably given rise to Melampsora and Bubakia. Crossopsora and Cronartium do not seem to be as closely related as usually assumed. It is likely that Ravenelia has given rise to Sphaerophragmium, Hapalophragmium, Dicheirinia, Diorchidium and Uromycladium, rather than the reverse, while Uromycladium appears to be the parent stock for Pileolaria and Maravalia. Within the grass rusts of the genera Puccinia and Uromyces a natural arrangement would begin with species having nearly colorless urediospores with scattered pores and indehiscent telia and gradually extend to species having such advanced characters as pigmented, equatorial pored urediospores and erumpent dehiscent telia.
5.	The data are in agreement with the general belief that evolution in the rusts has been toward simplification and reduction.