Heteropteran chromosomes are holokinetic; during mitosis, sister chromatids segregate parallel to each other but, during meiosis, kinetic activity is restricted to one pair of telomeric regions. This meiotic behaviour has been corroborated for all rod bivalents. For ring bivalents, we have previously proposed that one of the two chiasmata releases first, and a telokinetic activity is also achieved. In the present work we analyse the meiotic behaviour of ring bivalents in Pachylis argentinus (Coreidae) and Nezara viridula (Pentatomidae) and we describe for the first time the chromosome complement and male meiosis of the former (2n = 12 + 2m + X0, pre-reduction of the X). Both species possess a large chromosome pair with a secondary constriction which is a nucleolus organizer region as revealed by in-situ hybridization. Here we propose a new mode of segregation for ring bivalents: when the chromosome pair bears a secondary constriction, it is not essential that one of the chiasmata releases first since these regions or repetitive DNA sequences adjacent to them become functional as alternative sites for microtubule attachment and they undertake chromosome segregation to the poles during anaphase I.

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A cytological analysis of eight Argentinean species of Eryngium L. (Saniculoideae, Apiaceae) is carried out. Karyotypes of eight species and original chromosome counts for three of them (indicated with an asterisk) are notified: E. coronatum*Hook.Schlechtd. (2n=6x=48; 26m+ 4m-sm +14sm + 4st). The first three species belong to the Section Foetida, while the remaining five, belong to Panniculata. Both Sections are easily differentiated by morphology; our chromosomal study shows that these sections can also be recognized karyologically. All the species, except E. nudicaule (x=7), present x=8 which is the most common basic chromosome number in the genus and in the subfamily Saniculoideae. The karyotype analysis made on the eight species mainly shows metacentric and submetacentric chromosomes differing in proportion between species; only E. pandanzfolium and E. mesopotamicum show subtelocentric pairs. These two species only differ in the color of their inflorescences; besides, differences in their karyotypes were negligible. These facts agree with the suggestion that they would be varieties of the same species. Two different phenomena seem to have occurred during karyotype evolution in the genus Eryngium: aneuploidy within Section Foetida, and polyploidy within Section Panniculata.