MEIOSIS IN THE LEEK (ALLIUM-PORRUM L) REVISITED .1. PROPHASE-I PAIRING

Meiotic chromosome pairing of the tetraploid species Allium porrum, th e cultivated leek, was analysed by electron microscopy of 83 surface-s pread nuclei in the late zygotene to early diplotene interval of proph ase I, from four different varieties. Prophase I quadrivalent frequenc y, at 71%, marginally but significantly exceeds the two-thirds expecte d on a simple random end-pairing model, suggesting that more than two autonomous pairing sites occur, in some tetrasomes at least. This patt ern of synaptic behaviour is consistent with an autotetraploid status, but comparison with other tetraploids, including other Allium species , indicates that Allium porrum may be a weak segmental allopolyploid d isplaying limited preferential homologous pairing. The incidence of pa iring partner switches (PPSs) in prophase I quadrivalents is relativel y low; 90% of all analysed quadrivalents had only one or two PPSs. The positional distribution of PPSs along quadrivalents was distinctly un even with peaks in mid-chromosome arms and reduced frequencies around centromeres and near the ends. The four different varieties of leek an alysed were remarkably similar in their meiotic behaviour despite thei r diverse breeding history, but individual plants within varieties dis played substantial variation in quadrivalent and PPS frequencies.