Observations on unique sexual reproduction in Chlamydomonas proboscigera Korshikov in Pascher 1927

Abstract

A unique opportunity to study the sexual reproduction of Chlamydomonas proboscigera was presented when this species reached high densities in the Vaal River, South Africa during 2019. A peculiar and unique way of sexual reproduction in C. proboscigera was revealed for the first time and differs from earlier knowledge on the sexual reproduction of this species. Evidence of copulating gametes obtained during the study proved that C. proboscigera can also reproduce sexually by oogoniogamy and not only through isogamy, as previously believed. Gametes of different sizes copulated with each other and in addition to previously described copulation sites, results of this study indicated that gametes may attach to each other at different sites on the cell surface, including at their posterior ends, the posterior end of one gamete can attach to the side of the other gamete (laterally) or the posterior end of one gamete may attach sub-apically to the other gamete. Data on environmental variables at the time of sexual reproduction showed that oogoniogamy occurred under clear water conditions with low temperatures, sufficient nutrient concentrations, and slightly alkaline pH levels. Low temperatures may have played a role in inducing sexual reproduction to form resistant overwintering stages in the form of zygospores.