Separate Female and Male Meioses

Summary

The purpose of this advanced-topics chapter is to examine what is known about meiosis for Pinus, other genera within the Pinaceae family and to a lesser extent, other conifers and gymnosperms. Meiosis is the process by which the chromosomal complement is reduced by half. The resulting haploid spores, male and female, develop into multicellular gametophytes. Male and female meioses are both followed by cell division and subsequent growth of multicellular gameto-phytes. As a general rule, meiosis reshuffles existing genetic variation, generates de novo variation and ensures genome stability. But each taxon has its own recombination modification system and here the particulars are presented for Pinus, other members of the Pinaceae family and a few close relatives. Strict diploidy is a safe assumption for the Pinaceae. Oddly, most members of the Pinaceae have a uniform karyotype: 12 pairs of metacentric chromosomes. Meiosis I is characterized by few chiasmata per chromosome. Male and female meiosis in Pinus spp. are divergent, occurring at different times and producing different tetrad types. Meiotic recombination rates are higher in male gametes than female gametes. An consequence of heterospory, or separate male and female spores, is meiotic divergence between the sexes.