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Meiotic recombination between homologous chromosomes is vital for maximizing genetic variation among offspring. However, sex-determining regions are often rearranged and blocked from recombination. It remains unclear whether rearrangements or other mechanisms might be responsible for recombination suppression. Here, we uncover that the deficiency of the DNA cytosine methyltransferase DNMT1 in the green algaChlamydomonas reinhardtiicauses anomalous meiotic recombination at the mating-type locus (MT), generating haploid progeny containing bothplusandminusmating-type markers due to crossovers withinMT. The deficiency of a histone methyltransferase for H3K9 methylation does not lead to anomalous recombination. These findings suggest that DNA methylation, rather than rearrangements or histone methylation, suppresses meiotic recombination, revealing an unappreciated biological function for DNA methylation in eukaryotes. 

The transition of life from single cells to more complex multicellular forms has occurred at least two dozen times among eukaryotes and is one of the major evolutionary transitions, but the early steps that enabled multicellular life to evolve and thrive remain poorly understood. Volvocine green algae are a taxonomic group that is uniquely suited to investigating the step-wise acquisition of multicellular organization. The multicellular volvocine species Volvox carteri exhibits many hallmarks of complex multicellularity including complete germ-soma division of labor, asymmetric cell divisions, coordinated tissue-level morphogenesis, and dimorphic sexes-none of which have obvious analogs in its closest unicellular relative, the model alga Chlamydomonas reinhardtii. Here, I summarize some of the key questions and areas of study that are being addressed with Volvox carteri and how increasing genomic information and methodologies for volvocine algae are opening up the entire group as an integrated experimental system for exploring the evolution of multicellularity and more. 

Transitions between separate sexes (dioecy) and other mating systems are common across eukaryotes. Here, we study a change in a haploid dioecious green algal species with male- and female-determining chromosomes (U and V). The genus Volvox is an oogamous (with large, immotile female gametes and small, motile male gametes) and includes both heterothallic species (with distinct male and female genotypes, associated with a mating-type system that prevents fusion of gametes of the same sex) and homothallic species (bisexual, with the ability to self-fertilize). We date the origin of an expanded sex-determining region (SDR) in Volvox to at least 75 Mya, suggesting that homothallism represents a breakdown of dioecy (heterothallism). We investigated the involvement of the SDR of the U and V chromosomes in this transition. Using de novo whole-genome sequences, we identified a heteromorphic SDR of ca 1 Mbp in male and female genotypes of the heterothallic species Volvox reticuliferus and a homologous region (SDLR) in the closely related homothallic species Volvox africanus , which retained several different hallmark features of an SDR. The V. africanus SDLR includes a large region resembling the female SDR of the presumptive heterothallic ancestor, whereas most genes from the male SDR are absent. However, we found a multicopy array of the male-determining gene, MID , in a different genomic location from the SDLR. Thus, in V. africanus , an ancestrally female genotype may have acquired MID and thereby gained male traits.