CHAPTER 35 Achlya Nees von Esenbeck Nova Acta Phys.–Med. Acad. Caes. Leop.–Carol. Nat. Cur. 11:514. 1823 Hydronema Carus, Nova Acta Phys.–Med. Acad. Caes. Leop.–Carol. Nat. Cur. 11:514. 1823. Monoecious or dioecious. Sporangia fusiform, naviculate, cylindrical, or clavate; renewed sympodially or by basipetalous development and cymose branching. Spores mono- or dimorphic; when the latter the two flagella are quickly evanescent; primary spores at discharge encysting and clustering at the exit orifice in a spherical, hollow mass that subsequently may fall away from the sporangium; cysts floating free and then germinating to produce laterally biflagellate, reniform planonts, or remaining in the spherical cluster and then releasing the secondary planonts; in some species, the secondary sporangia discharge the spores in a dictyucoid or thraustothecoid manner, or the spores germinating in situ in an aplanoid fashion. Gemmae present or absent; functioning as sporangia, or oogonia, or producing new hyphae directly upon germination. Oogonia lateral on stalks or various lengths, or terminal, intercalary or sessile; variously shaped, but predominantly spherical or pyriform; sometimes reverting to the production of hyphae, or proliferating. Oogonial wall pitted or unpitted; smooth or ornamented on the outer surface, smooth or irregular on the inner. Oospores centric, subcentric, subeccentric, or eccentric; occasionally aborting; one to many in an oogonium; germinating by producing a mycelium directly or by a germ tube bearing a terminal sporangium. Antheridial branches present or absent, or reduced to a hypogynous cell; when present, diclinous, monoclinous, androgynous, or exigynous. Antheridial cells predominantly tubular or clavate; attached apically, laterally, or in a digitate fashion. Type species: Achlya prolifera Nees von Esenbeck, Nova Acta Phys.-Med. Acad. Caes. Leop.–Carol. Nat. Cur., 11:514. 1823. The genus Achlya has a blemished early history. From the figures accompanying the description of Bysus aquatica in Florae Danicae (O. F. Müller, 1782:6, pl. 896) it seems likely that the genus was first known under this binomial. The illustrations of this “alga” do not show sporangia or oogonia, but the clavate ends depicted on some of the filaments recall one of the stouter Achlyas. In 1819, a representative of the genus was accorded yet another binomial, Vaucheria aquatica, this proposed by Lyngbye. It is not clear whether Agardh (1824) was aware of Nees von Esenbeck’s work, for he reduced Byssus aquatica, Vaucheria aquatica, Schrank’s (1798) Conferva piscium, and Carus’ Hydronema to synonymy with Leptomitus clavatus, but made no mention of Achlya. In 1949, Kützing included Hydronema as a synonym of Saprolegnia ferax, but as the organism named by Carus was clearly an Achlya, it is obvious that Kützing’s concept of Saprolegnia also included Achlya. 473 Various proposals to divide the genus into subgenera or subgroups have appeared in the literature. An early attempt at such a division was that by Schröter (1893), in which two subgenera were recognized, Euachlya (for those species with smooth oogonia), and, for the ornamented Achlya spinosa, Acanthachlya. Coker (1923) separated the genus into four subgenera, Centroachlya (centric oospores), Euachlya (eccentric oospores), Glomeroachlya (branched oogonial stalks), and Thraustoachlya (spore release by some sporangia in a thraustothecoid manner). Centroachlya and Euachlya were further subdivided. Three subgenera, erected on the basis of oospore structure, ere proposed by Johnson (1956b). Two of these, Centroachlya and Subcentrica, certainly must be discarded since it is abundantly clear that centric and subcentric oospores can occur in the same species, and even in individual oogonia. Cejp (1959:127, 163, 176) reduced Coker’s and Johnson’s subgenera to synonymy with sectional designations – Prolifera, Apiculata, and Racemosa – that he created. Although he did not recognize subgenera, Dick (1973) carefully bundled the various species of the genus into groups based upon such characters as egg structure, racemose or glomerulate arrangement of oogonia, and the nature of oogonial wall ornamentations. The most extensive infrageneric segregation of the genus Achlya seems to be that adopted in 1954 by Naumov. He divided the genus into four subgenera, Proliferae, Hypogynae, Saprolegniopsis, and Thraustoachlya. The largest subgenus, Saprolegniopsis was further divided into two sections, Spinosae and Leiotheca, with the latter split into two subsections, Polyandrae and Brevipedes. Finally, Naumov segregated the subsection Brevipedes into the “divisions” De Baryanae and Racemosae. This detailed fracturing seems unnecessarily complicated. None of the infrageneric schemes of classification appears, in retrospect, to offer any proof of natural relationships. This being so, the retention (or introduction) of additional names provides only a convenience, and we see no reason to perpetuate a scheme which has no more value than this. Accordingly, we do not recognize subgeneric groupings. Members of the genus have been used extensively in physiological and biochemical studies (Chapters 16, 19, 20), and in ecological work (Chapters 4, 5). Perhaps the genus is best known for the work which has been done with its dioecious species on the hormonal control of sexual reproduction (Chapter 21). Some taxonomic implications have been extracted from analyses of DNA base composition (Green and Dick, 1972). A few species have been shown (Win-Tin and Dick, 1975) to have gametangial meiosis. The taxonomy of the genus is difficult; representatives of the genus do not always fall into place readily. This has, of course, been recognized for some time, and was first pointed out – with specifics – by Moreau and Moreau (1935a). Members of the genus producing smooth oogonia with eccentric oospores have long been notoriously difficult, and their identification often doubtful. Evidence from an analysis of several hundred collections of these Achlyas has prompted a severe revision of this section of the genus, much as Seymour (1970) did for the ferax complex of Saprolegnia. 474 Ordinarily, recognizing Achlyas in baited cultures or on organic substrates is done with ease, but there are exceptions that are sure to be encountered. A few species – A. dubia, for instance – practice thraustothecoid and dictyucoid spore discharge particularly from the secondarily produced sporangia. It is essential, therefore, that young cultures be examined so that the nature of discharge of the first-formed sporangia can be determined. Assignment of individuals to the genus is on the basis of these primary sporangia. By the inclusion into Achlya of two species formerly assigned to de Bary’s (1888) Aplanes, we have admitted taxa that consistently produce sporangia in such small numbers and so rarely that the species effectively are seen only as asporangiate forms. The two species, A. treleaseana and A. androgyna, are recognized easily by their dolioform, intercalary oogonia (these often develop only after 4–7 days in gross cultures), and appear to be restricted to acid bog habitats. See also, remarks under Protoachlya and Brevilegnia. Key to the species of Achlya 1. Thallus monoecious when isolated, although some cultures capable of cross-conjugating (interspecific mating) . 2 1. Thallus dioecious, being potentially antheridial or oogonial donors, when isolated; but some cultures capable of self-conjugation and expressing it on isolation . 43 2. Oospores centric, subcentric, or both . 3 2. Oospores eccentric . 17 3. Sporangia sparse or abundant, but present in initial growth period of isolates; special culture conditions not required to induce sporangia . 4 3. Sporangia extremely rare, and not present in initial growth period of isolates; may be induced only by special culture conditions . .16 4. Oogonial wall densely ornamented . 5 4. Oogonial wall predominantly smooth, but some oogonia may be very sparsely ornamented . 9 5. Oogonial wall ornamentations mammiform or predominantly so . A. radiosa (p. 481) 5. Oogonial wall ornamentations not mammiform or only very rarely and inconspicuously so . 6 6. Antheridial branches lacking . A. stellata (p. 484) 6. Antheridial branches present, though sometimes sparse . 7 7. Oogonial wall ornamentations papillate . 8 7. Oogonial wall ornamentations predominantly stout, lobed, truncate, indented, broadly bifurcate, 475 or wart-like; occasionally long-conical or long-cylindrical . A. spinosa (p. 484) 8. Oogonia positioned in a racemose fashion on short stalks producing closely attending androgynous antheridial branches . A. colorata (p. 486) 8. Oogonia positioned at irregular intervals on short or long stalks that are often coiled or twisted; antheridial branches monoclinous or androgynous . A. papillosa (p. 489) 9. Antheridial branches strictly diclinous, and often deliquescing . 10 9. Antheridial branches some combination of monoclinous, androgynous, and diclinous, but never strictly the latter; seldom if ever, deliquescing . 11 10. Oospores small, predominantly 22–28 µm in diameter . .A. oblongata var. oblongata (p. 490) 10. Oospores large, predominantly 36–49 µm in diameter . A. oblongata var. gigantica (p. 493) 11. Oospores centric . A. oligacantha (p. 494) 11. Oospores subcentric, or both subcentric and centric . 12 12. Oospores subcentric . 13 12. Oospores both centric and subcentric . 15 13. Oospores large, predominantly 44–52 µm in diameter . A. megasperma (p. 496) 13. Oospores small, predominant size