Rapid Evolution of a Sexual Reproduction Gene in Centric Diatoms of the Genus Thalassiosira

Diatoms are the most species-rich group of phytoplankton known. Conservative estimates suggest that tens of thousands of different species of diatoms are distributed throughout marine and freshwater ecosystems (27). Explosive diversification of diatom species has therefore occurred over the last 200 million years (27, 30). Typically, tens to perhaps hundreds of species of diatoms comprise the phytoplankton community of any given body of water. Under most circumstances, diatoms are likely to be essentially indifferent to whether neighboring cells are the same or different species. During sexual events, however, the ability of a given species to distinguish between itself and all other species becomes critical. The onset of sexual reproduction in diatoms is commonly coupled to control of cell size. Due to physical and developmental constraints associated with generation of the silica frustule, each mitotic division results in the formation of two daughter cells of different sizes, one that is the same size as the parent and one that is slightly smaller. Thus, over successive generations the mean cell size of a diatom population decreases (26, 37). Interestingly, only relatively small cells within a population respond to environmental signals and undergo sexual reproduction, an event that ultimately restores cell size (10). Consequently, multiple species of diatoms may undergo sexual reproduction simultaneously in a single body of water (reviewed in reference 11). In centric diatoms, flagellated sperm formed during sexual events must distinguish not only between vegetative cells and egg cells still encased within their frustule but also between vegetative and egg cells of different species. The molecular basis of species-specific gamete recognition during external fertilization has been examined in marine invertebrates such as abalone (42, 45), sea urchin (31), and teguline gastropods (17). A common feature of sexual recognition proteins appears to be rapid diversification of amino acid sequences in closely related species (33, 43). In many instances, strong selection for sequence variation, known as positive Darwinian selection, appears to occur. The unicellular freshwater algal genus Chlamydomonas is the only phytoplankton genus in which evolution of a sex-related protein has been examined (12). The C. reinhardtii protein, MID, is required for gamete differentiation, and the sequences from two closely related species display dramatic differences, although positive selection does not appear to underlie the evolution of this protein. Ultimately, rapid diversification of sexual recognition proteins is expected to lead to speciation, although it remains unclear what forces underlie the evolution of new species (12, 45). We recently identified in the centric diatom Thalassiosira weissflogii a gene family, composed of Sig1, -2, and -3, whose transcription is highly upregulated during the onset of sexual reproduction. The proteins encoded by these genes appear to be part of the extracellular matrix and have been hypothesized to play a role in mediating sperm-egg recognition (1). Our initial goal in the present study was to determine whether Sig homologues could be identified in other species of centric diatoms and whether upregulation of these genes could serve as a molecular marker for the occurrence of sexual reproduction in field populations. In this work, we found that the gene on which we focused, Sig1, is a multicopy gene that appears to be undergoing rapid divergence both within and between species, a feature that has come to be expected for genes encoding proteins involved in sexual recognition.