Heteropycnosis in the Autosome Segments of Ceriagrion coromandelianum (Odonata)

Abstract

THE sex-determining mechanism in dragon-flies consists, as a rule, of XO—XX chromosomes, and the X-chromosome in the spermatocyte divides equationally in the first, and passes undivided to one pole or the other in the second, meiotic division^1–4. In this respect, the X-chromosome of the male germcell in the Odonata behaves as in the Heteroptera, in which group also it usually divides in the first meiotic division only. We find that the chromosomes of the damsel-fly, Ceriagrion coromandelianum, exhibit another important feature in respect of which they resemble the hemipteran chromosomes; that feature is the presence of Feulgen-positive interconnecting threads joining the chromosomes at the spermatogonial metaphase, and also at the first and second meiotic metaphase. Such interconnecting chromatic threads are known to exist in many hemipterans^5–8. Associated with the presence of these threads is a high degree of heteropycnosis of autosome segments. At pachytene the paired chromosomes carry deeply stained, condensed knobs. These heterochromatic segments occur in all chromosome pairs. The interconnecting threads, in our opinion, are derived from the chromosome matrix. It is well known that the protein framework of the heterochromatic chromosomes (or chromosome segments) differs from that of euchromatic chromosomes⁹. It is likely that the nucleoprotein matrix synthesized by these segments is cohesive⁷ and that any fortuitous contact and subsequent separation between chromosomes result in the drawing out of the matrix substance into fine threads.