CHAPTER 8. Morphogenesis: the Spore

The first recognizable mention of spores in the Saprolegniaceae is in Gruithuisen’s account (1821:445, pl. 38, figs. 14-16) of “seeds” in “compartments” at the ends of filaments of “Conferva” ferax. Hannover’s exposition (1842) of the nature of spores and spore discharge in a watermold was far more detailed: records of daily observations on motility, encystment, and germination. It was Unger (1843, 1844), however, who first gave an accurate sketch of percurrent renewal, spore discharge and encystment. He saw one flagellum in the lateral groove of a secondary spore, and concluded that water flowed through this depression under the action of “vibrating organs.” Thuret (1845) noted that the cilium (flagellum) insertion described by Unger was analogous to the “organs of locomotion” (two apically attached flagella) on ”Conferva” spores (Thuret, 1843: pl. 10, figs. 1-3). In 1851 a full description of sporulation in Saprolegnia ferax (as Achlya prolifera) appeared in an account by Pringsheim. He observed spores that were by chance attached to one another by their flagella, and decided that these structures were “copulation threads.” The concept of two swimming stages in the watermolds -“diplanetism”-had its beginning with Leitgeb (1869-70) and his description of Diplanes saprolegnioides. Later work was to show that, in fact, multiple swarmings and encystments may occur (Salvin, 1940, for example). Accordingly, the terms monoand diplanetic, and monoand dicystic (Coker, 1935), are not strictly applicable to the watermolds as E. A. Bessey (1950) so perceptibly argued. Ordinarily, members of the Saprolegniaceae are thought of as expressing spore dimorphism to a very advanced degree. This is true of species of Saprolegnia, but in other genera one stage is suppressed. The primary spores are pyriform (de Bary, 1852), and are provided with two subapical flagella which, in motion, are oppositely directed. The secondary, reniform planonts (described as “bean”or “kidney”-shaped, terms that Weston, 1917, firmly objected to) emerging from encysted primary ones are laterally biflagellate, with the posterior flagellum being the longer of the two (Crump and Barnton, 1966). Scanning electron micrographs published by M. F. Brown and Brotzman (1979:27, fig. 2) show the anterior flagellum of Aphanomyces cochlioides to be noticeably shorter and stouter than the posterior one. Further observations are needed to confirm whether this size difference occurs in all species of the genus. Any effects of SEM preparative techniques must be accounted for in such studies. Schussning (1948, 1949) introduced two terms to refer to these patterns of flagellar insertion: subakrokont for the subapically attached pair, and pleurokont for the laterally positioned ones. The secondary spores of Aphanomyces cochlioides and Saprolegnia sp. are not strictly reniform (Ho et al., 1968). The anterior portion is flattened, and the two flagella are attached nearest to the forward end of the cell. The spore is perceptibly flattened on the side on which the lateral groove appears. The secondary planont is the more vigorous swimmer of the two. It is this fact that perhaps led H. M. Ward (1883), Humphrey (1893), and Höhnk (1933) to conclude that the “purpose” of the primary spore of a watermold was to evacuate the sporangium, while that of the secondary one was to disperse the organism. Coker’s