Unique state of sexual dimorphism of crossing-over in diplotene spermatocytes and oocytes of Mesocricetus brandti, a species with neonatal oogenesis.

Sexual dimorphism of recombination has been held by classic genetic theory to disfavor the heterogametic sex. Assessment of chiasma frequencies at the diplotene stage of meiosis has been used as a valid measure of this concept and in many species has revealed, as expected, an increased frequency in female vs. male germ cells. Mesocricetus brandti, a species currently used in investigations of gonadal regression, photoperiods, and hibernation, was found by this measure to be an exception to this rule, with an average of 29.81 chiasmata in spermatocytes and 23.16 in oocytes. Sites of crossing-over unique to each sex were also detected. Oogenesis occurs in this species as a postnatal phenomenon and is suggested as a possible critical factor for the exceptional recombinant behavior.