POSTREDUCTION OF THE X-CHROMOSOME AND COMPLETE CHIASMA INTERFERENCE IN THE LAMPYRIDAE (COLEOPTERA)

Abstract

Five species representing four genera in three of the five tribes comprising the Lampyridae of North America, the only genera known cytologically, have nine pairs of autosomes and an XX (♂): XO (♀) sex-determining mechanism. A single individual of one species, Pyractomena angulata, carried supernumerary chromosomes, which varied in number in different cells from zero to four. The X-chromosome of males is strongly, positively heteropycnotic during early prophase; the X-bivalent of females is indistinguishable from the autosomal bivalents at pachytene. The complements of the five species are indistinguishable but within the complement there are considerable differences both in size and in the position of the centromeres. In all five species the chiasma frequency per cell is without exception nine: univalents not being encountered, the recombination index for the family, as known cytologically, stands uniformly at 81. Chiasma interference, being complete, must therefore extend across the centromere. The supernumeraries, which approximate one-half the size of the smallest regular autosome, have a chiasma frequency of ca. 0.25 per chromosome, or one-half that of the smallest autosome. They thus demonstrate that, contrary to the evidence provided by the regular complement, a length-frequency relationship exists among the latter but is masked by the totality of chiasma interference. As in other beetles, the centromeres are localized, but the lampyrids are unique in having postreductional division of the X in males. Other insect groups with postreduction have, or are reputed to have, diffuse centromeres: one, the Odonata, is invariably postreductional.