16 



PLASMODIOPHORALES 



of the so-called gametic nuclei then divides (fig. 92), 

 during which division it undergoes a chromatin re- 

 duction and forms a variable number of reduction 

 bodies ( !). ]Meiosis is followed almost at once by 

 karyogamy. Apparently all but two nuclei degen- 

 erate (fig. 93), and the two remaining ones fuse to 

 form a synkaryon (fig. 9i). Prowazek's account of 

 reduction is not very clear, and his drawings of the 

 process do not clarify the accompanying description. 

 It is accordingly difficult to determine from his con- 

 fusing account the duration of the respective diploid 

 and haploid generations in Plnsmodiophora. Accord- 

 ing to him, the diploid phase is api)arently quite 

 short (text-fig. 1). 



Prowazek's account was refuted by ]\Iaire and 

 Tison ('09) who failed to find any evidence of plas- 

 mogamy and karyogamy following cleavage. They 

 nevertheless believed that sexual fusions occur in 

 Plasmodiophora and postulated that it might take 

 place between two amoebae from germinating spores. 

 Pavillard ('10) rejected this view and considered it 

 more plausible that karyogamy occurs in the Plasmo- 

 dium shortly before meiosis. presumably following 

 a coalescence of amoebae. His theory, however, re- 

 lates to the Plasmodiophoraceae as a whole rather 

 than to Plasmodiophora specifically. Winge con- 

 curred with Maire and Tison's view and assumed 

 that the motile cells from resting spores copulate in 

 pairs to form small myxoplasma which penetrate the 

 host and develop into plasmodia. The diploid phase 

 persists until the second sporogonic division where 

 reduction occurs, according to Winge (text-fig. 2). 

 This text-figure is also representative of his view 

 concerning alternation of generations in all genera 

 of the Plasmodiophorales. Lutman, Chupp, and 

 Milovidov ('31) were uncertain about the time and 

 place of plasmogamy and karyogamy, but believed 

 that they must occur at some stage on the grounds 

 that a reduction in chromosome number takes place 

 during the first sporogonic division (PL 3, fig, 63- 

 73). Miss Terby ('21') postulated that fusion occurs 

 outside of the host cell between pairs of zoospores, a 

 view which Nawaschin had accepted by 192i. 



P. M. Jones ('28) described and figured the for- 

 mation of two types of gametes from germinating 

 spores in culture. In some cases a large pyriform uni- 

 flagellate gamete is formed in germination, while in 

 others the content of the spore emerges, grows, and 

 then divides into as many as 20 minute gametes. 

 Both types of gametes may fuse in pairs and form 

 zygotes, but sometimes a large number of micro- 

 gametes which have not completely separated re- 

 unite to form a plasmodium. .Tones furthermore re- 

 ported that during the chromidial stage a whole Plas- 

 modium may break up into gametes which subse- 

 quently fuse in pairs, as is shown in text-figure 3. 

 However, his account of the life cycle of P. Brassicae 

 is so unorthodox and confused that most later work- 

 ers have seriously questioned the accuracy of his ob- 

 servations. As noted elsewhere. Cook and Schwartz 

 ('30) maintained that the small flagellate cells pro- 

 duced in zoosporangia are gametes which fuse in 



pairs either in the root hairs of the host or after 

 migrating into the cortex. They regarded these small 

 zoospores as comparable to the minute gametes re- 

 ported by .Jones. Cook and Schwartz, however, never 

 observed actual fusion, and their hypothesis is based 

 entirely on the observation of zoospores lying side 

 by side in pairs and the subsequent occurrence of 

 binucleate amoebae. Obviously, neither of these phe- 

 nomena are conclusive proof of fusion. According to 

 these workers, the zygote thus formed develops into 

 the sporogenous plasmodium, and reduction occurs 

 during the first sporogonic division. As is shown in 

 text-figure -t, the diploid generation thus embraces 

 only the zygote and sporogenous ])lasmodium. Cook 

 and Schwartz were uncertain whether the gametes 

 come from tlie same or from different gametangia. If 

 sex is genotypically segregated at meiosis, the rest- 

 ing spores, zoospores, haploid plasmodia, gametan- 

 gia, and gametes are of two types, as is indicated in 

 text-figure 5. Fedorintschik confirmed Cook and 

 Schwartz's report of fusion of gametes from zoo- 

 sporangia or gametangia but believed that it occurs 

 later, following a period of vegetative budding 

 within the host. As noted before, he also believed that 

 two reductions occur in P. Brassicae — one during the 

 first division of the sporangium nucleus and another 

 at sporogenesis. Fedorintschik may have been in- 

 fluenced by a previous report by Cook of two simi- 

 lar reductions in Lif/iiiera. If two reductions occur, 

 obviously there must be two nuclear fusions, but 

 Fedorintschik reported only one. No additional or 

 more convincing evidence of sexuality in Plasmodio- 

 phora has since been presented as far as the author 

 is aware, and the question thus remains in this un- 

 certain state. It will doubtless remain thus until in- 

 tensive monospore studies have been made. 



In Telramyxa, Cook ('33) reported that "swarm 

 cells" fuse in pairs at their anterior ends as they 

 migrate from cell to cell and thus form amoeboid 

 zygotes in which karyogamy soon occurs. These ob- 

 servations were apparently made from slides of fixed 

 material furnished by Prof. O. V. Darbishire and do 

 not relate to fresh material. No other data on sex- 

 uality in this genus exist so far as the author is 

 aware. According to Cook, T. parasitica has a dis- 

 tinct alternation of haploid and diploid generations, 

 as is illustrated in text-figure 6. 



Nothing definite is known about sexuality in Soro- 

 sphaera. Cook ('33, p. 198) stated that amoebae 

 from resting spores fuse in pairs and form amoeboid 

 zygotes, but this statement is not based on observa- 

 tion. No one lias yet reported actual observation of 

 gametic fusion. However, inasmuch as reduction is 

 said to occur at s))orogcnesis, most workers have 

 nevertheless assumed that plasmogamy and karyog- 

 amy take place at some stage of develoi)mtnt. Webb 

 found no evidence of plasmogamy but rejiorted that 

 the chromosome number is doubled during the transi- 

 tional phase. He thus concluded that karyogamy oc- 

 curs at this stage, as Home had ])reviously described 

 for Spongospora. According to \\'ebb, the diplophase 

 of Sorosphaera is very short and includes only the 



