How do the Rhizophydiales reproduce asexually?
The body of a chytrid is called a thallus (plural = thalli). The thallus of a typical Rhizophydium species consists of two parts:
an absorptive branching rhizoidal system that contains no nuclei;
a multinucleate sporangium that ranges in shape from spherical,to oval, to pear-shaped, and to multi-lobbed (Fig. 1).
(Fig. 1) Thallus consisting of a spherical sporangium and branched rhizoids (Fig. 2)
Mature sporangium with two discharge pores appearing as domes, each plugged with gelatinous material
Sporangium releasing a cloud of numerous zoospores through one of several discharge pores
When the thallus of Rhizophydium species is fully grown, the multinucleate sporangium cleaves out unwalled, single nucleate zoospores, each bearing a single posteriorly directed flagellum.
Zoospores exit the sporangium, either through one to several inoperculate openings in the sporangial wall (Fig. 2) or through a pore that opens with an operculum (a flap-like opening to the discharge pore). In some species, zoospores are released as a mass, remain quiescent at the discharge pore, and then swim away (Fig. 3). In others, zoospores are released one at a time through a discharge pore or tube.
The zoospore has to use its own stored food reserves (lipids and glycogen) as it swims until it attaches to a suitable host or substrate, absorbs its flagellum, produces a wall around itself, and grows a germ tube that penetrates the substrate.
This stage is called the germling (Fig. 4). The germ tube of the germling becomes the rhizoidal axis of the thallus. The rhizoidal system may be sparsely or extensively branched (Figs. 1,5). The encysted zoospore portion of the germling expands and becomes the multinucleate sporangium (Figs. 1,5).
|Fig. 4 Germling consisting of encysted zoospore and tubular germ tube, which is branching||Fig 5
The zoospore cyst expands to form the sporangium and the germ tube grows and branches forming the rhizoidal system
How do zoospores locate a suitable host or substrate during asexual reproduction?
Zoospores of parasitic chytrids use light and chemical cues to locate hosts. Zoospores of Rhizophydium littoreum, a parasite of marine green algae, are positively phototactic toward blue light, a mechanism that might assure that zoospores swim to the photic zone where its host resides. Zoospores of both R. littoreum and B. dendrobatidis exhibit chemotaxis to specific sugars, proteins and amino acids, also a mechanism by which zoospores might detect signals to potential hosts.
How do Rhizophydium species reproduce sexually?
There are only a few reports of sexual reproduction among species of Rhizophydium, and they are all of algal parasites. The first report was made by Scherffel (1925) in R. granulosporum, a parasite of the green algae, Tribonema.
Although several mechanisms for sexual reproduction have been described, they all involve a receptive thallus and contributing thallus or cyst. By morphological convention, the contributing thallus is considered the male gamete/ gametangium and the receptive thallus the female gamete/ gametangium.
After transfer of the contents from the contributing thallus to the receptive thallus, the receptive thallus becomes a zygote and differentiates into a thick-walled resting spore containing large lipid globules in the cytoplasm. The resting spore enlarges; but the contributing thallus and receptive thallus remain distinct, often with the contributing thallus persisting as an appendage on the resting spore.
After a dormancy period, the resting spore germinates directly as a sporangium or secondarily as a prosporangium, with the budding out of a sporangium from which zoospores are formed and released, continuing the life history of the chytrid. Current understanding is that meiosis occurs in the resting spore prior to germination.
There are two basic schemes described for sexual reproduction among Rhizophydium species, and the differences are in the details of whether or not the receptive thallus is attached to the host substrate.