84 



PLASMODIOPHORALES 



ing bodies of Amaurochaete, Reticularia, Trichia, 

 Stemonitis, and Didymium. In these genera karyog- 

 amy is followed by synapsis, and as the spores are 

 delimited, one mitosis, the heterotypic division, oc- 

 curs. Tliis first meiotic division is followed by a long 

 rest period of the spore, and the second or homeo- 

 typic division is delayed until the first mitosis in the 

 germinating spore, according to Jahn. Similar ob- 

 servations were reported by Kriinzlin (07) and 

 Vouk (11) for species of Trichia and Arcyria, but 

 these were later found to be incorrect by Jahn in 

 1911. For the first time in the Myxomycetes he found 

 that haploid myxamoebae of Physarum didermoidcs 

 fuse in ]iairs to form the zygote. Plasmogamy is fol- 

 lowed shortly by karyogamy. The diploid zygote 

 may engulf hajjloid amoebae, with the result that 

 haploid and diploid nuclei may be found in the young 

 Plasmodia. Likewise, zygotes may fuse with each 

 other to form larger plasmodia, but fusion of the dip- 

 loid nuclei does not occur. ]Meiosis takes place during 

 the last division in the young sporangium and is not 

 followed by a homeotypic division. Jahn ('33) re- 

 ported the same type of meiosis in Badhamia iitricu- 

 laris, and subsequently persisted in this view on the 

 time and nature of reduction division in the Exospo- 

 reae and Endosporeae. 



Pinoy (08) concluded from his culture experi- 

 ments that Didymium nigripes is heterothallic and 

 forms -|- and — myxamoebae which in turn give rise 

 to -|- and — Plasmodia. Sporangia are formed only 

 wiien both types of plasmodia are mixed. It is not 

 certain that Pinoy used monospore cultures, and be- 

 cause of this his results have been seriously ques- 

 tioned by Kniep ('28) and Schiinemann ('30). Sku- 

 pienski ('17— '28) also reported heterothallism in D. 

 ni(/ripes and D. difforme. In 1928 he asserted that 

 the sjjores of D. difforme are unisexual and that no 

 sporangia will develop in monospore cultures. Ac- 

 cording to him the plasmodium arises by the fusion of 

 two myxamoebae of opposite sex (text-figure It). 

 Other myxamoebae may unite with the zygote, but 

 the gametic nuclei remain separate and divide mitot- 

 ically in the young plasmodium. The daughter nu- 

 clei later unite in pairs and fuse in the older plas- 

 modium. wliile those whicli fail to find partners de- 

 generate. Meiosis occurs during the last two divisions 

 in the sporangium, according to Skupienski. 



In the same year Wilson and Cadman showed in 

 Reticularia Lycoperdon that haploid motile gametes 

 fuse in pairs by their posterior ends to form a zygote 

 (text-figure 17). Other gametes may coalesce with 

 the zygote, but their nuclei divide amitotically, de- 

 generate, and are digested by the zygote. Karyogamy 

 of the gametic nuclei follows shortly after the coal- 

 escence witli the non-functional gametes, and meiosis 

 occurs during the last two divisions in the sporogenic 

 protoplasm. Miss Clayley ('29) refuted Skupienski's 

 contention of heterothallism in D. difforme, showed 

 that the s))ores are bisexual, and secured sporangia in 

 monosjjore cultures (text-figure l.'j). She also found 

 that plasmogamy takes place between motile gametes 

 instead of myxamoebae, as claimed by Skupienski. 



Schiinemann likewise secured plasmodia in mono- 

 spore cultures of Skupienski's own D. difforme and 

 thus refuted the latter's contention of heterothallism. 

 In D. .ranihopus, however, neither plasmodia nor 

 sporangia were formed in monospore cultures. In 7^. 

 niciripes, Schiinemann found that several haploid 

 myxamoebae coalesce to form plasmodia but their 

 nuclei remain separate until the plasmodia become 

 older (text-figure 16). Karyogamy eventually oc- 

 curs, and reduction is accomplished during the two 

 divisions preceding spore formation. ScIiUnemann 

 tlius concluded that a true antithetic alternation of 

 generations occurs in I), nic/ripes. Cadman ('31), 

 however, found that karyogamy occurs shortly after 

 plasmogamy, and that the diploid zygote can ingest 

 zoospores and haploid myxamoebae and coalesce 

 with other zygotes. She nevertheless confirmed Schii- 

 nemann on meiosis. In the same year Howard re- 

 ported fusion in pairs of motile gametes in Physarum 

 polycephalum and expressed tlie belief that plasmo- 

 gamy is followed at once by karyogamy. Abe ('33, 

 'SI) likewise found fusion of motile gametes in 

 Fulif/o septica, Erionema aureum, D. nic/ripes, P. 

 crateriforme, and Stemonitis fusca. The gametes 

 were found to be isomorphic but differ physiologi- 

 cally, as has been noted previously. 



In D. nigripes, Stosch ('35, '37) reported the dis- 

 covery of two forms, Z). eunigripes and D. aantho- 

 pus, which are hetero- and homotliallic, respectively. 

 In D. eunigripes, sexuality is well defined, while D. 

 jranthopus is apogamous. Didymium squamulosum 

 and Physarum cinereum were also reported to be 

 apogamous, the first report of which Jahn ('36) 

 characterized as fantastic. Jahn furtlier refuted 

 Stosch's report of heterothallism in D. eunigripes 

 and claimed that tlie failure of the gametes to fuse 

 and form plasmodia and sporangia in Stosch's mono- 

 spore cultures was due to the fact that tliey had not 

 gone through tlie encystment and rest period which 

 are necessary before fusion occurs. For sexual spe- 

 cies of the Didymaceae, Stosch reported that motile 

 gametes fuse in pairs to form zygotes, which in turn 

 fuse with other zygotes in the formation of large 

 ])lasmodia. Plasmogamy of gametes is apparently 

 followed shortly by karyogamy. Onlj' one vegetative 

 mitosis occurs before cleavage in the sporangium, 

 and meiosis takes place in the spore, according to 

 Stosch. Separation of homologous chromosomes may 

 occur in the first and second divisions. Stosch thus 

 supported Olive's and Gilbert's contention that meio- 

 sis occurs in the spore instead of before cleavage in 

 the sporangium. In ajjogamous species, he reported 

 that fusion may occur between amoeboid as well as 

 motile gametes, and tliat instead of meiotic divisions 

 in the spore, one or ))erliai)s two vegetative divisions 

 occur which arc followed by amitosis. 



It is apparent from this survey that there are 

 marked differences in observations and interpreta- 

 tions concerning karyogamy, meiosis, alternation of 

 generations, and sex segregation in the Myxomycetes 

 as well as in the Plasmodioi)horales. Nonetheless, 

 certain fundamental similarities do exist, and if the 



