CONIDIOSPORE EVOLUTION
While it is obviously unwise to draw too rigid conclusions about the evolution of a structure from a study of a limited number of existing types, the genus Pythium provides a number of examples in which the asexual reproductive organs apparently show a transition from a simple zoosporangium to a true conidiospore. It must be realized, however, that in citing the following species as stages in such a series the reproductive organs may in certain instances germinate by different methods under differing conditions, and that many species of Pythium exhibit both zoospores and conidiospores, the former being produced under wet and the latter under dry conditions.
In the genus Phytophthora, which is included in the family Pythiaceae, the zoosporangia develop from the vegetative mycelium and form zoospores which are kidney shaped and escape directly from the sporangium without the formation of a vesicle. Such a condition is not found in any species of Pythium, for this is one of the characters used to separate the genera. In Pythium gracile, however, the zoosporangia are filamentous and differ but little from the ordinary hyphae. Each, however, produces a long, narrow vesicle which terminates in a spherical swelling and resembles a zoosporangium in appearance. It is this s\yollen sporangium-like structure which grows out of the host cell and in \yhich the zoospores are developed, and from which they are finally liberated as reniform, biflagellated bodies.
In Pythium debaryanum, as we have seen, the zoosporangia are spherical in shape and develop a vesicle into which the contents of the zoosporangium pass and in which the zoospores are differentiated. Under dry conditions, however, conidiospores are formed which are similar in shape and size to the zoosporangia. In fact, we may consider that these conidiospores are really zoosporangia which form no zoospores but germinate by a germ tube.
In Pythium intermedium both zoosporangia and conidiospores occur. The latter germinate by one or more germ tubes, while the former produce zoospores in a vesicle. In both cases, however, these reproductive bodies are formed in chains so that the conidiospores appear to be abstricted in a similar way to the zoosporangia of Cystopus, but they do not give rise to zoospores as in that genus.
In Pythium vexans zoosporangia are rarely formed, but the asexual reproductive organs behave as conidi,ospores, the germ tube being produced in place of the papilla of a zoosporangium.
Finally in Pythium ultimum zoosporangia are never formed, but spherical terminal conidiospores are- produced which germinate by a germ tube in the usual way.
A similar series can be traced in the genera which comprise the Peronosporaceae, and we may briefly outline the series here.
In the genus Basidiophora the sporangia are borne in clusters from the top of a clavate sporangiophore. Each zoosporangium is spherical and possesses a definite apical papilla through which biflagellated, reniform zoospores ultimately emerge.
In the genus Plasmopara the sporangiophore is complexly branched and the zoosporangia develop terminally on the ultimate branches. Usually the contents of the zoosporangium divide up into separate zoospores which escape, but sometimes the structure forms a germ tube and behaves as a conidiospore.
In the genus Bremia the sporangiophores are complexly branched, each branch terminating in a saucer-shaped expansion bearing a number of sterigmata. On each of these a single conidiospore is borne. This conidiospore possesses an apical papilla, and it is through this that the germ tube emerges. Zoospores are never formed.
Finally in the genus Peronospora itself the branched sporangiophores terminate in long, fine sterigmata, each of which bears a single conidiospore. There is no apical papilla, and the germ tube is produced from an indeterminate point on the side of the spore.
From the above evidence, therefore, it seems reasonable to consider that the conidiospore, at least in this group, has been derived from a zoosporangium, the contents of which produce a germ tube instead of dividing up into a number of separate zoospores. This change from motile zoospores to conidiospores may have been brought about as an - adaptation to drier conditions in which motile zoospores were less successful than aerially distributed conidiospores.
