Introduction to the cytotaxonomy of the odonate genus Argia Rambur (Zygoptera: Coenagrionidae)

In the hitherto studied taxa (12) of the genus the <5 haploid numbers range from 13 to 19, viz.; n=!3: moesta (Hag.), nahuana Calv., translata Hag.; n=!4: fumipennis atra Gloyd, f.fumipennis .(Burm.), f. violacea (Hag.), funebris (Hag.), immunda (Hag.), sedula (Hag.), vivida (Hag.); n=!9: apicalis (Say), tibialis (Ramb.). An additional element occurs in some populations of f. atra and f. violacea (n= 15), but only in the former is its bivalent structure more or less apparent. Save for translata and for the n= 15 sets of f. alra and f. violacea, mchromosomes are lacking. Aside from one case in Pseudagrion, this is the only coenagrionide genus in which spp. occur with n lower than the family type number (14); n=!9 is the highest chromosome number yet recorded in a dragonfly. The low-n complements are of a secondary origin and are characterized by an increased chiasma frequency. The reduction of n is due to the obligatory fusion of 2 pairs of the primary karyotype. The n= 15 complements are not obligatory and are due to fragmentation of one of the original pairs (bivalents). The elements of the n=!9 spp. have the usual Argia size; the origin of the high-n complements is unknown, and their TCL is significantly higher than that of the n= 14 sets. The variation of n does not appear significant on the subgeneric level. The evolutionary significance of the exceptionally high variation in the recombination potentials is discussed and it is concluded that the genus is in the midst of an intense evolutionary process, and is far from having reached its final adaptive level. In this feature Argia is unique among the higher taxa of the Order that so far have been studied cytologically.