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chloroplast E.R. which was the remains of the food vesicle membrane of the host.
Although the above evolutionary scheme is discussed in one sequence, it is probable that two membranes of chloroplast E.R. evolved at least three times, with one line leading to the Chlorarachniophyta, a second to the Cryptophyta, and the third (or more) leading to the Heterokontophyta and Prymnesiophyta.
The algae with two membranes of chloroplast E.R. are:
Chlorarachniophyta: chloroplast derived from a green alga, chlorophyll a and b present, nucleomorph between inner and outer membrane of chloroplast E.R.
Cryptophyta: Chlorophyll a and c, phycobiliproteins, nucleomorph between inner and outer membranes of chloroplast E.R., starch in grains between inner membrane of chloroplast E.R. and chloroplast envelope, periplast inside plasma membrane, tripartite hairs on flagella.
Heterokontophyta: tripartite hairs on anterior tinsel flagellum, posterior whiplash flagellum, chlorophyll a and c, fucoxanthin, storage product usually chrysolaminarin in vesicles in cytoplasm.
Prymnesiophyta (haptophytes): two whiplash flagella, haptonema present, chlorophyll a and c, fucoxanthin, scales common outside cell, storage product usually chrysolaminarin in vesicles in cytoplasm.
Chlorarachniophyta
These algae (Fig. V.2) represent an intermediate stage in the evolution of two membranes of chloroplast endoplasmic reticulum. This group has a small number of green amoebae that have ingested green algal cells in the past, with the green algal cells evolving into endosymbionts within the amoeba host (Fig. V.3) (Hibberd and Norris, 1984). A nucleomorph or reduced nucleus occurs in the green algal symbiont. The reduced nature of the nucleomorph implies that some of the functions originally coded by the DNA of the endosymbiont nucleus have been taken over by the nucleus of the host amoeba. The chloroplast (e.g., endosymbiont chloroplast) contains chlorophyll a and b and is surrounded by four membranes. The innermost two membranes are those of the chloroplast envelope of the endosymbiont. The next membrane is the plasma membrane of the endosymbiont and the outer membrane represents the food-vacuole membrane of the amoeba host. Thus, the algae in the Chlorarachniophyta represent an intermediate stage in the evolution of the chloroplasts of some of the algae in the Heterokontophyta.
Chlorarachnion reptans is a marine amoeba that forms large plasmodia with the individual cells linked by a network of reticulopodia (Geitler, 1930; Hibberd and Norris, 1984). The cells are naked and contain a number of lobed chloroplasts, each with a central pyrenoid (Fig. V.3). Four
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Fig. V.4 Chlorarachnion reptans. (Adapted from Hibberd and
Norris, 1984; Grell, 1990.)
The cells move over the reticulopodia and ingest other algal cells and bacteria as a food source.
Under nutrient deprivation, the star-shaped vegetative cells become resting cells by retracting their reticulopodia, rounding up and secreting a thin cell wall (Grell, 1990). The resting cells apparently rely principally on photosynthate from the chloroplasts as a food source. The resting cells germinate to star-shaped vegetative cells under favorable conditions. Zoosporogenesis occurs by a resting cell dividing twice to produce four zoospores, each with a single flagellum wrapped around the cell body (Fig. V.1(c) and V.4). The zoospores settle to produce the starshaped vegetative cells. Sexual reproduction occurs when a non-motile female gamete is approached by a motile, star-shaped, male gamete. The gametes fuse producing a zygote that germinates into a star-shaped vegetative cell (Grell, 1990).
REFERENCES
Calderon-Saenz, E., and Schnetter, R. (1989). Morphology, biology, and systematics of Cryptochlora perforans (Chloroarachniophyta), a phagotrophic marine alga. Pl. Syst. Evol. 163:165–76.
