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Chromosome numbers vary greatly in Orgyiu, from low numbers, like n = 11 in 0. thyellina and n = 14 in 0. antiqua, to a high number, n = 30, as in 0. recens and 0. ericue. Meiotic synapsis was regular in 0. thyellina and 0. 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. Chromosome numbers in animals range from n = 1 in the nematode Parascaris univalens and the ant Myrmecia pilosula to n % 220 in the butterfly Lysandra atlantica (BOVERI 1899; CROSLAND and CROZIER 1986; DE LESSE 1970). Thus increases and/or decreases must have taken place during evolution. Dissociation rather than polyploidization is regarded the main source of increases in chromosome number of bisexually reproducing animal species (WHITE 1973). Decreases on the other hand are thought to be caused by chromosome fusions. The crude limitations to increases and decreases are dictated by the dimensions of the spindle. The chromosome arms must be small enough not be cut by the cleavage furrow but the chromosome number must not exceed the number that fits into the equatorial plate and gets attached to the spindle (WHITE 1973). Although it is conceivable that the number of linkage groups is under selective |
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