PACHYTENE PAIRING AND METAPHASE-I CONFIGURATIONS IN A TETRAPLOID SOMATIC LYCOPERSICON-ESCULENTUM X LYCOPERSICON-PERUVIANUM HYBRID

In the tetraploid somatic hybrid between the diploid Lycopersicon spec ies L. esculentum (tomato) and L. peruvianum, synaptonemal complexes f ormed quadrivalents in 73 of the 120 sets of four chromosomes (60.8%) in 10 cells studied in detail at pachytene. Of these, 43 had one pairi ng partner exchange, 22 had two, and 8 had three, very close to a Pois son distribution. The points of pairing partner exchange were concentr ated at the middle of the two arms. The frequency per arm corresponded with physical arm length. There was a sharp drop around the centromer e, and pericentric heterochromatin had a slightly lower probability of being involved in pairing partner exchange than euchromatin. The chro mosomes align before pairing and there are several points of pairing i nitiation, with concentrations at or near the ends and the centromere. From zygotene to late pachytene the quadrivalent frequency decreased considerably. At late pachytene it was lower than expected with the ob served high frequency of pairing partner exchange. Fairing affinity be tween species was only slightly lower than affinity within species, in spite of considerable genetic differentiation. The frequency of recom bination nodules increased from early to late zygotene and then decrea sed strongly to full pachytene. There is a highly significant negative correlation between percent pairing and SC length. At metaphase I the frequency of quadrivalents was 0.444, and branched quadrivalents were rare, probably caused by interference and restriction of chiasma form ation to distal euchromatin. Metaphase I quadrivalent frequency is a r elatively good indication of pairing affinity in this material.