•10 



THE SIMPLE HOLOCARPIC BIFLAGELLATE PHYCOMYCETES 



character. There are no size differences between the 

 male and female nuclei, so that it is impossible to 

 distinguish them on this basis, according to Barrett 

 and McLarty. Both of these workers found nuclei 

 in pairs, and Barrett figured a few stages of what he 

 believed to be nuclear fusion (figs. 80-82). Although 

 he did not find conclusive evidence of nuclear fusions 

 in pairs, he nonetheless believed it occurs. McLarty, 

 on the other hand, failed to observe fusion and held 

 that the occasional occurrences of nuclei in pairs 

 may be merely fortuitous. Therefore, the problem of 

 the type of karyogamy, i.e., whether the numerous 

 gametic nuclei fuse in pairs or all but one pair de- 

 generate, in Olpidiopsis still remains to be solved. 

 That plasmogamy of male and female thalli is not 

 always essential to resting spore development is 

 evident in the species which are partly or wholly 

 parthenogenetic. In 0. Achlyae, McLarty observed 

 cases in which only part of the male protoplast 

 passed into the female th alius. Occasionally one 

 male may "serve" two female thalli, and in some 

 instances as many as two to eight empty male thalli 

 have been found attached to a single resting spore. 

 Apparently in these instances there are supernu- 

 merary male nuclei following plasmogamy, but 

 whether or not the unmated ones degenerate is not 

 known. Obviously, sexual reproduction in Olpidiop- 

 sis presents numerous unsolved cytological prob- 

 lems, and until these have been solved it will be im- 

 possible to determine how closely Olpidiopsis is re- 

 lated to the higher Oomycetes. 



It is also probable that male thalli are capable of 

 developing androgenetieally into resting spores. At 

 least this is suggested by the small spore shown in 

 figure 108. In some species the male thallus is at- 

 s tached to the female by an attenuated neck or canal 

 (figs. 143, 144, 149, 150, 156), through which the 

 male protoplast passes during plasmogamy. As in 

 other organisms this neck is generally referred to as 

 a conjugation or fertilization canal. Sometimes the 

 male thalli may occur in tandem (fig. 149) but 

 whether or not the content of the terminal one passes 

 through the adjacent companion cells to reach the 

 female in such cases is not known. 



Sex Determination 



It has been generally assumed by most mycolo- 

 gists that species of Olpidiopsis are heterothallic, 

 inasmuch as the resting spores are formed usually 

 by fusion of thalli of unequal size. However, the 

 monospore culture experiments of McLarty and 

 Slianor on O. Achlyae and O. varians have discred- 

 ited this belief, and it now seems that most, if not 

 all, Olpidiopsis species are homothallic or haplosy- 

 noecious as was earlier suggested by the author ( '39) 

 and McLarty ('39b). As noted elsewhere McLarty 

 and Shanor found that sexually formed resting 

 spores may occur readily in cultures propagated 

 from a single zoospore. Similar results will probably 

 be secured from other species when they have been 



studied in monospore cultures. In light of what is 

 known to occur in haplonts, meiosis probably oc- 

 curs during the first division of the diploid (?) nu- 

 clei in the germinating resting spore, but whether or 

 not sex is genotypically differentiated at this stage 

 is not absolutely certain inasmuch as McLarty and 

 Slianor did not make monozoospore cultures from 

 germinating resting spores. However, the fact that 

 a single zoospore from a sporangium will give rise 

 to cultures which later form male and female thalli 

 shows that it carries the potentialities of both sexes. 

 If sex is genotypically determined at meiosis in the 

 germinating resting spore the resultant zoospores 

 would be male and female in equal numbers and de- 

 velop into sporangia of opposite sexes, which is con- 

 trary to the results obtained by McLarty and 

 Shanor. According to their data sex in Olpidiopsis 

 appears to be determined phenotypically at some 

 stage in the haploid generation. Olpidiopsis Ach- 

 lyae and 0. varians accordingly seem to be hap- 

 losynoecious. At which stage in the life cycle sex 

 differentiation occurs is not known. As has been 

 pointed out above incipient zoosporangia as well 

 as the male and female thalli are multinucleate and 

 quite similar in appearance, and until differentia- 

 tion occurs it is impossible to tell which type of re- 

 productive structure is going to develop from them. 

 It seems that all thalli in the early stages are poten- 

 tial male and female cells which under certain ex- 

 ternal environmental and internal conditions become 

 differentiated and develop into gametes. 



Cellular Relations Between Host and Parasite 



As has been noted elsewhere, all species of this 

 genus appear at present to be obligate parasites 

 with a limited host range. So far they have not been 

 successfully grown on synthetic media, although 

 Diehl ('35) was able to bring zoosporangia to ma- 

 turity on agar. According to his observations, the 

 maturation stages of zoosporangia are not depend- 

 ent on the presence of the host. All Olpidiopsis spe- 

 cies which parasitize members of the Saprolegniales 

 and Pythium usually cause marked local hyper- 

 trophy of the infected hyphae but do not induce sep- 

 tation except in the case of Pythium. In the latter 

 host the supporting hyphae may occasionally be de- 

 limited from the remainder of the mycelium by cross 

 walls (figs. 112, 113). The swellings in the host 

 hyphae may vary markedly in shape and size, and 

 may be terminal, intercalary, or in some cases pro- 

 ject out as lateral diverticula. Most species which 

 parasitize algae cause little or no hypertrophy, but 

 O. zopfii and O. appendiculata may induce local 

 swellings which are two to four times the normal 

 diameter of the algal filaments. Infection by O. 

 Oedognoriorum leads to the formation of a conspicu- 

 ous plug of cellulose by the host cell (fig. 131 ) at the 

 point of entry of the germ tubes. 



In cases of infection by O. Achlyae, the penetra- 

 tion of the germ tube and entrance of the parasite 



