KARYOTYPE EVOLUTION BY CHROMOSOME FUSION IN THE MOTH GENUS ORGYIA

Chromosome numbers vary greatly in Orgyia, from low numbers, like n = 11 in O. thyellina and n = 14 in O. antiqua, to a high number, n = 30, as in O. recens and O. ericae. Meiotic synapsis was regular in O. the llina and O. antiqua; 11 and 14 normal bivalents, respectively, were f ound in meiosis. The paired homologues displayed homologous chromomere patterns. In the species hybrid between antiqua and thyellina, many s ynapsed chromosome segments were found in meiosis. This indicates suff icient segmental homology between chromosomes of the two species altho ugh the paired pachytene chromosome segments rarely displayed similar chromomere patterns. Chromosomes switched pairing partners, thus formi ng multivalents, linked by chiasmata in males, and long synaptic chain s in the achiasmatic females. Multivalent formation is understood as t he consequence of a separate evolution of the two species from a speci es with a high chromosome number. Multiple chromosome fusions resulted in similarly low chromosome numbers but different segmental compositi ons of the chromosomes in the two species.