Genes specifically expressed in sexually differentiated female spheroids of Volvox carteri

In search of new tractable diatoms for experimental biology

Diatoms are a species-rich group of photosynthetic eukaryotes, with enormous ecological significance and great potential for biotechnology. During the last decade, diatoms have begun to be studied intensively using modern molecular techniques and the genomes of four diatoms have been wholly or partially sequenced. Although new insights into the biology and evolution of diatoms are accumulating rapidly due to the availability of reverse genetic tools, the full potential of these molecular biological approaches can only be fully realized if experimental control of sexual crosses becomes firmly established and widely accessible to experimental biologists. Here we discuss the issue of choosing new models for diatom research, by taking into account the broader context of diatom mating systems and the place of sex in relation to the intricate cycle of cell size reduction and restitution that is characteristic of most diatoms. We illustrate the results of our efforts to select and develop experimental systems in diatoms, using species with typical life cycle attributes, which could be used as future model organisms to complement existing ones.

Sexual pheromone induces diffusion of the pheromone-homologous polypeptide in the extracellular matrix of Volvox carteri

The C-terminal domain of pherophorin II is homologous to the sexual pheromone of Volvox carteri and is released from other domains during sexual induction. Green fluorescent protein fused to the C terminus of pherophorin II was located at the extracellular matrix directly surrounding the gonidium, the final target of the sexual-induction signal.

A small cysteine-rich extracellular protein, VCRP, is inducible by the sex-inducer of Volvox carteri and by wounding

The green alga Volvox carteri represents one of the simplest multicellular organisms: it is composed of only two cell types, somatic and reproductive. Volvox is capable of both vegetative and sexual reproduction. Sexual development of males and females is triggered by a sex-inducer at concentrations as low as 10(-16) M. By differential screenings of cDNA libraries, a novel gene was identified that is under the control of this sex-inducer and that encodes a small cysteine-rich extracellular protein, named VCRP. Analysis of the VCRP polypeptide sequence suggests ten disulfide bonds and a dimetal-binding capacity. VCRP mRNA is detectable in males and females approximately 1 h after the spheroids' first contact with the sex-inducer, but transcription is restricted to the somatic cell-type. mRNA and protein synthesis is triggered not only by the sex-inducer, but also by wounding. VCRP does not share significant sequence similarity with any known protein sequence, but a potential EGF-like calcium-binding motif and a potential plant metallothionein family-15 motif have been identified. The characteristics of VCRP suggest a function as a signal transducer molecule, an extracellular second messenger from somatic cells to reproductive cells, or a role within the stress response.

A posttranslationally regulated protease, VheA, is involved in the liberation of juveniles from parental spheroids in Volvox carteri

The lineage of volvocine algae includes unicellular Chlamydomonas and multicellular Volvox in addition to their colonial relatives intermediate in size and cell number. In an asexual life cycle, daughter cells of Chlamydomonas hatch from parental cell walls soon after cell division, while Volvox juveniles are released from parental spheroids after the completion of various developmental events required for the survival of multicellular juveniles. Thus, heterochronic change in the timing of hatching is considered to have played an important role in the evolution of multicellularity in volvocine algae. To study the hatching process in Volvox carteri, we purified a 125-kD Volvox hatching enzyme (VheA) from a culture medium with enzymatic activity to degrade the parental spheroids. The coding region of vheA contains a prodomain with a transmembrane segment, a subtilisin-like Ser protease domain, and a functionally unknown domain, although purified 125-kD VheA does not contain a prodomain. While 143-kD VheA with a prodomain is synthesized long before the hatching stage, 125-kD VheA is released into the culture medium during hatching due to cleavage processing at the site between the prodomain and the subtilisin-like Ser protease domain, indicating that posttranslational regulation is involved in the determination of the timing of hatching.