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. thyellina and O. antiqua; 11 and 14 normal bivalents, respectively, were found in meiosis. The paired homologues displayed homologous chromomere patterns. In the species hybrid between antiqua and thyellina, many synapsed chromosome segments were found in meiosis. This indicates sufficient segmental homology between chromosomes of the two species although the paired pachytene chromosome segments rarely displayed similar chromomere patterns. Chromosomes switched pairing partners, thus forming multivalents, linked by chiasmata in males, and long synaptic chains in the achiasmatic females. Multivalent formation is understood as the consequence of a separate evolution of the two species from a species with a high chromosome number. Multiple chromosome fusions resulted in similarly low chromosome numbers but different segmental compositions of the chromosomes in the two species.