PERONOSPORALES 1019 



Sexual Reproduction 



Sexual reproduction in the Pythiaceae is oogamous, typically with 

 only a single egg formed in each oogonium. The oogonia and antheridia 

 may be developed on the same hypha, the antheridium originating from 

 the oogonial stalk (monoclinous) (Figs. 86 A, H, p. 1030; 87 B, p. 1032) 

 or on different hyphae and the antheridium not arising from the oogo- 

 nial stalk (Fig. 86 J) (diclinous). 1 If the relationship of the two is such 

 that the antheridium is surmounted by the oogonium or is formed 

 either within the stalk or at its apex (Fig. 87 D, p. 1032) it is called 

 "hypogyny." If, as in certain species of Phytophthora, the antheridium 

 encircles the oogonial stalk, as a consequence of the growth of the 

 oogonial rudiment through that of the antheridium (Fig. 90 I, p. 1059), 

 this peculiar situation is known as "amphigyny." For the most part, 

 however, members of the family have the antheridium applied in 

 lateral association with the oogonium, either near to or distant from 

 the stalk (Fig. 86 A). This last condition is termed "paragyny." The 

 characters of the antheridium (the number, shape, size, place of origin, 

 and so forth) are usually of high taxonomic value in determination of 

 species. Dioecism ("heterothallism") is rare, if not absent, in most 

 genera. According to Arnaudow (1925), it occurs in Zoophagus insidians. 



The development and structure of the sex organs are best known in 

 Pythium. In the aquatic P. adhaerens and P. cmgustatum, for example 

 (Sparrow, 1931b), the oogonia originate as terminal or intercalary 

 swellings of the hyphae, which gradually increase in size until they are, 

 when finally mature, spherical bodies cut off by cross walls from the 

 adjacent hyphae. The antheridia in these two species are usually diclinous 

 in origin, occasionally monoclinous, and arise as short lateral expanded 

 hyphal branches, which clasp the oogonium early in its formation. From 

 two to five antheridia are generally in contact with an oogonium, the 



oogonium; E, oospore to whose oogonium three diclinous antheridia are 

 attached; F, oospore starting to germinate; large globule is broken up and 

 wall is beginning to be resorbed; G, oospore with wall nearly completely 

 resorbed; H, branched hyphal system produced by germinating oospore. 

 (Sparrow, 1931b) 



1 Following usage by Middleton (1943). 



