From diploids to allopolyploids: The emergence of efficient pairing control genes in plants

Polyploidy has played a major role in the evolution of higher plants. Precise control of chromosome pairing is vital for conferring meiotic regularity, and hence reproductive stability in allopolyploids. In this review, we examine whether strong evidence has accumulated for the presence and activity of pairing control genes in different allopolyploid species that are entirely bivalent forming and that display a strict disomic inheritance. We show that very good evidence has been adduced in Triticum species, Avena sativa, Festuca arundinacea, Brassica napus, Gossypium hirsutum, and G. barbadense, and in amphidiploids related to the diploid species Lolium perenne, L. multiflorum, and L. rigidum. More circumstantial evidence has been obtained for polyploids in the genera Aegilops, Hordeum, Nicotiana, and Coffea, which have received far less attention than the other species. Although these pairing regulators seem to control different processes operating throughout the premeiotic interphase and the meiotic prophase, little is known about their precise mode of action. We present three hypotheses that have been proposed to explain the origin and evolution of pairing control genes; none of them has been supported by direct evidence, and the origin of most pairing suppressors is still unknown. Accordingly, the study of pairing control genes is still an important task for understanding the stabilization and establishment of allopolyploid species.