MYTHS AND MECHANISMS OF MEIOSIS

A comparative analysis of the meiotic sequence in a wide variety of plants and animals indicates there is no convincing evidence that: premeiotic pairing plays any role in the synapsis of homologues; heterochromatic association facilitates homologous pairing; chiasmata ever form within segments which are positively heteropycnotic at zygotenepachytene; localization of chiasmata depends on prior localization of pairing or on the occurrence of euchromatin-heterochromatin boundaries; prior association of centromeres plays any role in determining co-orientation; any form of supra-chromosomal organization exists involving permanent association between the members of a haploid complement and unequal progeny ratios recovered from structurally modified Drosophila complements arise as a consequence of distributive pairing. There is evidence that: interlocking of bivalents can occur regularly in species with a chiasma frequency sufficiently high to regularly produce ring bivalents and in which the chiasmata are localized to the ends of the bivalent; some forms of terminal association cannot represent terminalized chiasmata; U-type exchanges present at diplotene result from errors in crossing over; pairing and chiasma formation are not necessary for coorientation; and at least some types of elastic constrictions present at 1st metaphase represent extended nucleolar organizers.