A cytogenetic study of the leaf beetle genus Cyrtonus (Coleoptera, Chrysomelidae).

The chromosomes of ten species of Cyrtonus and the genome sizes of six are surveyed. Among the total of 15 chromosomally studied species, 11 have 2n = 28 chromosomes and a 13 + Xyp male meioformula, three have 2n = 40 and 19 + Xyp and one 2n = 46 and 22 + Xyp. All but one species with 28 chromosomes show only metacentric or submetacentric chromosomes, whereas the species with 40 and 46 chromosomes display some telocentrics or subtelocentrics, that are probably derived from the former by centric fissions. However, since the number of major chromosome arms is strikingly higher in these latter species (NF = 70 and 78) than in the 28-chromosome species (mostly NF = 56), other chromosomal rearrangements such as pericentric inversions or heterochromatin accretions could also be involved. The genome sizes display a narrow range, from 1C = 0.6-1.22 pg, and they are not significantly correlated with the chromosome numbers. Some possible factors implied in the rough chromosomal evolution of Cyrtonus are discussed in relation to a few other genera of the subfamily Chrysomelinae.