The Development and Cytology of Rhodochytrium

Abstract

Rhodochytrium does not appear in North Carolina until late in the spring; at first zoosporangia are most abundant, but late in the season only resting spores are found. The cysts are independent, not connected through their rhizoids. The zoospores are of the algal type and frequently contain starch grains, but are colorless except for the red anterior end; they are active for half a day or more, but seem to conjugate rarely except when confined in small amounts of fluid. The nature of the cyst (resting spore or zoosporangium) is determined on infection. The red pigment which is found at all stages of the life cycle is haematochrome or an allied lipochrome. Although the three races of Rhodochytrium appear to be geographically isolated and affect different hosts, no morphological differences were detected between them. The germ tubes do not enter the stomata, but penetrate the epidermis at any point, usually in the vicinity of a vascular bundle. The cysts, both resting and temporary, are uninucleate until full size is attained. Their rhizoids extend along the vascular bundles, mostly in the phloem elements, which they destroy, but they also send haustoria to the vessels of the xylem. When mature the resting spores have a two-layered cellulose exospore and a thick non-cellulose endospore; most of the reserve food is in the form of starch; the nuclei are considerably shriveled by the withdrawal of karyolymph. The starch grains are similar to those commonly seen in the higher plants. No plastids could be found, the starch grains appearing to be built up directly in the plasma. The flaring necks of the zoosporangia are stopped by characteristic turbinate or bell-shaped plugs. During the last mitoses there is a contraction which results in a pseudo-segmentation, but true segmentation appears to be brought about by the precipitation of membranes around the protospores. There is a deeply staining body at the base of the cilia of the zoospores which is connected with the nucleus. The primary nuclei, which reach the size of 50-60 μ, have enormous nucleoli and peculiar amorphous masses of chromatin like Synchytrium decipiens. In the first type of mitosis, the spindle, which is usually unipolar at first, is formed from coarse acicular fibers that appear within the nuclear cavity; it has no connection with the nuclear membrane. The spirem is formed from that part of the chromatin which lies in the equatorial region, the rest being cast out; it is frequently entirely within the spindle. The second type of mitosis presents no unusual features. No centrosomes or true asters were seen. Amitosis is rare and abnormal in the zoosporangia. Although superficially resembling Entophlyctis, Rhodochytrium is not closely related to any known Archimycete. But it appears to be closely related to the Protococcoideae through Phyllobium. The Phyllobiae show considerable similarity to Synchytrium in gross morphology. The cytology of Rhodochytrium bears a strong resemblance to that of Synchytrium. These resemblances suggest that Synchytrium was derived from protococcoid ancestors.