Volvocine cell walls and their constituent glycoproteins: an evolutionary perspective

Abstract

Similarities in the composition of the extracellular matrix suggest that only some species of the unicellular Chlamydomonas are closely related to the colonial and multicellular flagellated members of the family Volvocaceae. The cell walls from all of the algae in this volvocine group contain a crystalline layer. This lattice structure can be used as a phylogenetic marker to divide Chlamydomonas species into distinct classes, only one of which includes the volvocacean algae. Similarly, not all species of Chlamydomonas are sensitive to each other’s cell wall lytic enzymes, implying divergence of the enzyme’s inner wall substrate. Interspecific reconstitution of the crystalline layer is possible between C. reinhardtii and the multicellular Volvox carteri, but not between C. reinhardtii and C. eugametos. The hydroxyproline-rich glycoproteins (HRGPs) which make up the crystalline layer in genera which have a similar crystal structure exhibit many homologies. Interestingly, the evolutionarily distant cell walls of C. reinhardtii and C. eugametos also contain some HRGPs displaying a few morphological and amino acid sequence homologies. The morphological similarities between the flagellar agglutinins (HRGPs responsible for sexual recognition and adhesion during the mating reaction) and the cell wall HRGPs leads to the proposal of a superfamily from which novel HRGPs (designed for self-assembly/recognition) can constantly evolve. Just as variations in the wall HRGPs can lead to unique wall structures, new agglutinins facilitate sexual isolation of new species. Thus, the HRGPs could emerge as valuable phylogenetic markers.