Schizosaccharomyces japonicus Sexual Cycle
Schizosaccharomyces japonicus is a yeast species related to Schizosaccharomyces pombe. As such, it has an oblong cell shape (Figures 1-3) and divides mitotically by medial fission; i.e., a contractile ring in the middle of the cell (Figure 2 and 3) splits the organism into two daughter cells. After division, these daughter cells often remain attached forming a “V”-type configuration (Figure 4). Although the rod-type shape predominates, spherical cells are often seen as part of the population.
S. japonicus offers several advantages for our intended evolution experiment. It grows fast with a doubling time of about 1.8 h at 32°C. Its haploid genome of about 11.7 Mb, encoding 4,878 predicted genes, has already been sequenced (Rhind et al., 2011). Such a small reference genome facilitates the task of resequencing and reassembling. In addition, its three chromosomes sample many features found in higher eukaryotes, such as intron-interrupted genes, repetitive elements (10 families of gypsy-type retrotransposons), and epigenetics based on RNA interference, chromatin modifications, and genome-wide antisense regulation (Rhind et al., 2011). Accordingly, genomic variations observed in S. japonicus are expected to reflect general patterns of genome evolution found in eukaryotes.
One of the most important features of S. japonicus for our evolution experiment is that this yeast species has a very well defined sexual cycle and sporulates vigorously. The sexual line used in our experiment is the homothallic strain Sj4 (kindly donated by Professor Amar J. S. Klar; Klar, 2013). This strain switches between the two mating types (minus and plus), thus cells within the Sj4 population mate prolifically. The trigger of the sexual cycle is lack of nitrogen in the medium. Within a few hours following the nitrogen deficiency signal, Sj4 cells start to agglutinate (Figure 5). By 5-6 hours the agglomeration is massive, comprising the majority of cells in the population (Figure 6). Agglutination facilitates encounter between plus and minus cell-types and stimulates contact that will lead to the cell-to-cell fusions (Figures 7-10), a process known as conjugation. Cellular conjugation is seen in the context of the massive agglomerates, and is particularly visible at their borders (Figures 7 and 8). Interesting, because of the close contact cells are capable of fusing to more than one partner (specially visible in Figures 9 and 10). As the fusion often occurs at the cell poles, the conjugational partners form a very characteristic “bone” or “boomerang” shape (Figures 8-10). At this point cytological events take place, such as fusion of the two nuclei (Karyogamy), DNA replication, and then meiosis, involving crossing over between homologous chromosomes and ploidy reduction divisions. These events unfold from 6 to 10 hours after the sporulation was triggered. By 10-12 hours the cell agglomerates are completely dissipated and individual ascus are already discernible alongside conjugants (Figure 11). S. japonicus asci usually contain eight ascospores (Figures 11 and 12) as a result of one additional mitotic division of the four meiotic nuclei (Tanaka and Hirata, 1982). By 24 hours massive presence of asci can be seen alongside cells that have not mated (Figures 13-15). By this time it is common to observe the rupture of asci liberating individual spores in the medium (Figures 13-16). Those are oval or ellipsoidal shaped and often resemble a "chickpea" because of a characteristic protuberance at their surface (Figure 16). When exposed to a nutrient-rich medium, spores germinate by swelling and assuming the shape of a vegetative cell (Wickerham and Duprat, 1945). The mitotic reproductive cycle (Figures 1-3) is then resumed.
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