The karyotype of Risiocnemis incisa Kimmins, 1936 from Luzon, The Philippines (Zygoptera: Platycnemididae)

The <J chromosome number is 2n = 21, n = 11, incl. an m-pair and the univalent X. The complement is of a secondary origin, the result of a fusion of 6 elements of the primary, 2n d = 25 set. The fusion is obligatory in all cells and permanent in all stages of the mitotic cycle. This is the first case of a low-n complement in Platycnemididae. Chiasma frequency is not increased, hence the recombination index is extremely low. This situation favours the survival value and reproductive capability of incisa, makes the sp. fit to settle down over a wide geographic range, but it restricts its adaptability to substantial ecological diversity of habitats. Considering the deviation of the recombination index from the family level, coupled with structural affinities and geographic range of the genus, it is suggested that Risiocnemis is likely to represent a phylogenetically young offshot of the calicnemine stock.