The Occurrence and Phylogenetic Significance of Putative Placental Transfer Cells in the Green Alga Coleochaete

Abstract

Following fertilization, zygotes of the green alga Coleochaete orbicularis, which are retained on the haploid thallus, first enlarge, then become covered with a layer of vegetative cells. Light microscopy and high-voltage electron microscopy revealed the presence of localized wall ingrowths in vegetative cells adjacent to zygotes. These covering cells resemble the gametophytic placental transfer cells of embryophytes in their morphology, location, and time of development. If Coleochaete cells with wall protuberances function as do placental transfer cells of embryophytes, their presence is evidence that photosynthates may be transported between haploid thallus cells and zygotes. Thus, a nutritional relationship between different phases of the life cycle, similar to that which occurs in embryophytes, may also have evolved in green algae. This first report of putative placental transfer cells in a green alga supports Bower's (1908) ideas concerning the origin of land plant sporophytes and alternation of generations. The presence or absence of cells with wall ingrowths in several species of Coleochaete was correlated with estimates of zygote-plant area ratios.

Green Algae and the Origins of Multicellularity in the Plant Kingdom

“…The best-documented occurrence of matrotrophy in charophyte algae comes from studies of Coleochate in which the developing zygote remains associated with the vegetative thallus (Graham 1984). These studies identified regions of the vegetative maternal cells specifically at the zygote interface that appeared specialized for maximal cell-cell contact and nutrient transport (Graham and Wilcox 1983;Delwiche et al 1989). The molecular details of these junctions and how they form is not known, but their presence in advanced charophyte algae and similarity to embryophyte maternal-zygotic junctions suggests a common origin.…”

Sexual conflict and the alternation of haploid and diploid generations

“…Zygotes undergo substantial post-fertilization growth as they accumulate nutrients supplied by the maternal gametophyte. In C. scutata and its relatives, cells of the maternal gametophyte grow around and completely surround the zygote to produce a corticated zygospore that exhibits substantial post-fertilization growth (Graham & Wilcox 1983;Delwiche et al 2002). Multiple zygospores may be present simultaneously on a female gametophyte, but not all eggs produce mature zygospores.…”

Occurrence and evolutionary significance of RESISTANT CELL WALLS IN CHAROPHYTES AND BRYOPHYTES

“…The Charophyceae is the class of green algae that shares the greatest number of derived molecular, biochemical, and morphological characters with land plants (embryophytes) (Graham, 1993). This taxon includes the orders Chlorokybales, Klebsormidiales, Zygnematales, Coleochaetales, and Charales (Mattox and Stewart, 1984).…”

“…Generally known as "charophytes," these algae are regarded as the extant protists most closely related to embryophytes. A single origin of embryophytes is also supported by a suite of morphological autapomorphies for which independent origin of the whole character-state group is unlikely (Gra-ham, Delwiche, and Mishler, 1991;Graham, 1993). A single origin of embryophytes is also supported by a suite of morphological autapomorphies for which independent origin of the whole character-state group is unlikely (Gra-ham, Delwiche, and Mishler, 1991;Graham, 1993).…”