Morphological Development and Nuclear Behavior in the Genus Taphrina

SUMMARY

The developmental morphology and nuclear behavior of 5 species, which are considered to represent some of the major morphological types within the genus Taphrina, have been studied. The development of the mycelium by members of the genus has attained three distinct levels with regard to its location within the host tissue. These are the intercellular, subcuticular, and cell wall habits of development. Only members with the first two types have been studied at this time. These and the variations that exist among them may be summarized as follows:

1.	Intercellular mycelium that does not form a subcuticular layer, but instead bears asci at the tips of hyphal branches which grow outward between the epidermal cells, such as found in T. caerulescens and T. carnea;
2.	Mycelium which is at first intercellular, but becomes subcuticular and fragments to form a hymenium of separate ascogenous cells, such as found in T. deformans;
3.	Mycelium that is only subcuticular in habit and which remains intact throughout the development and maturation of the asci; (Not all of the cells enter into ascus formation, some remain sterile. Only 2 species, T. ulmi and T. celtii, exhibit this type of mycelial development.)
4.	Subcuticular mycelium which is sparingly branched and which remains intact during the early development of the ascogenous cells; (As in the above type, not all cells enter ascus formation. This type of mycelial formation may be found in T. populi-salicis.)
5.	Subcuticular mycelium that is richly branched and which fragments into a compact hymenium of ascogenous cells, such as found in T. carveri and T. betulae;
6.	Hymenia which are formed by subcuticular colonies of budding yeastlike cells which enlarge to form a compact layer of ascogenous cells; (Mycelium is only sparsely produced, functioning mainly in distributing the fungus over the leaf surface.)
7.	Mycelium that grows within the walls of the epidermal cells and locules in which the ascogenous cells and asci develop.

Three distinct patterns of nuclear behavior are now evident in the genus. They are as follows:

1.	Species with stalk cells in which the fusion nucleus undergoes an equational division preceding meiosis. One of the resulting nuclei migrates into the stalk cell and disintegrates while the other undergoes meiotic division and finally produces 8 ascospores. This type of nuclear behavior occurs in all species with stalk cells that have been studied with the exception of T. populi-salicis.
2.	The species Taphrina virginica, which lacks a stalk cell, possesses a nuclear cycle that is the same as that described above except that the fusion nucleus undergoes immediate meiosis and spore formation without a preceding equational division. The extent to which this type of nuclear cycle occurs among those organisms is not known, although it is suspected that it does occur in at least some other species which lack stalk cells.
3.	The species Taphrina populi-salicis has a nuclear behavior that is the same as the first type mentioned above, except that nuclear division does not stop after eight haploid nuclei are formed but continues indefinitely to produce a large number of ascospore nuclei. This is the only species in which this type of nuclear cycle is known to occur; however, it seems quite possible that the same phenomenon may occur in other species.