Factors underlying restricted crossover localization in barley meiosis

Meiosis, unreduced gametes, and parthenogenesis: implications for engineering clonal seed formation in crops

Meiosis and unreduced gametes. Sexual flowering plants produce meiotically derived cells that give rise to the male and female haploid gametophytic phase. In the ovule, usually a single precursor (the megaspore mother cell) undergoes meiosis to form four haploid megaspores; however, numerous mutants result in the formation of unreduced gametes, sometimes showing female specificity, a phenomenon reminiscent of the initiation of gametophytic apomixis. Here, we review the developmental events that occur during female meiosis and megasporogenesis at the light of current possibilities to engineer unreduced gamete formation. We also provide an overview of the current understanding of mechanisms leading to parthenogenesis and discuss some of the conceptual implications for attempting the induction of clonal seed production in cultivated plants.

Epigenetic control of meiotic recombination in plants

Meiotic recombination is a deeply conserved process within eukaryotes that has a profound effect on patterns of natural genetic variation. During meiosis homologous chromosomes pair and undergo DNA double strand breaks generated by the Spo11 endonuclease. These breaks can be repaired as crossovers that result in reciprocal exchange between chromosomes. The frequency of recombination along chromosomes is highly variable, for example, crossovers are rarely observed in heterochromatin and the centromeric regions. Recent work in plants has shown that crossover hotspots occur in gene promoters and are associated with specific chromatin modifications, including H2A.Z. Meiotic chromosomes are also organized in loop-base arrays connected to an underlying chromosome axis, which likely interacts with chromatin to organize patterns of recombination. Therefore, epigenetic information exerts a major influence on patterns of meiotic recombination along chromosomes, genetic variation within populations and evolution of plant genomes.

Synaptonemal complex extension from clustered telomeres mediates full-length chromosome pairing in Schmidtea mediterranea

In the 1920s, József Gelei proposed that chromosome pairing in flatworms resulted from the formation of a telomere bouquet followed by the extension of synapsis from telomeres at the base of the bouquet, thus facilitating homolog pairing in a processive manner. A modern interpretation of Gelei's model postulates that the synaptonemal complex (SC) is nucleated close to the telomeres and then extends progressively along the full length of chromosome arms. We used the easily visible meiotic chromosomes, a well-characterized genome, and RNAi in the sexual biotype of the planarian Schmidtea mediterranea to test that hypothesis. By identifying and characterizing S. mediterranea homologs of genes encoding synaptonemal complex protein 1 (SYCP1), the topoisomerase-like protein SPO11, and RAD51, a key player in homologous recombination, we confirmed that SC formation begins near the telomeres and progresses along chromosome arms during zygotene. Although distal regions pair at the time of bouquet formation, pairing of a unique interstitial locus is not observed until the formation of full-length SC at pachytene. Moreover, neither full extension of the SC nor homologous pairing is dependent on the formation of double-strand breaks. These findings validate Gelei's speculation that full-length pairing of homologous chromosomes is mediated by the extension of the SC formed near the telomeres. S. mediterranea thus becomes the first organism described (to our knowledge) that forms a canonical telomere bouquet but does not require double-strand breaks for synapsis between homologous chromosomes. However, the initiation of SC formation at the base of the telomere bouquet, which then is followed by full-length homologous pairing in planarian spermatocytes, is not observed in other species and may not be conserved.