Patterns of Variation in Chiasma Formation

Abstract

In Chorthippus chiasmata form at various points along the length of the chromosome. But even in cases like this, with so-called random chiasma formation, it has long been evident that the pattern is not completely random (see pg. 207). However, in some organisms chiasmata occur exclusively, or nearly so, in very localised regions of the chromosome. Two such types of localisation are known—proximal and distal. In the first of these, chiasmata are restricted to the neighbourhood of the centromere, while in the second they are confined to short regions near the non-centric ends of the chromosome (Table 2, Figs. 27–30). This localisation rarely appears to be an absolute nuclear condition affecting all chromosomes equally. Thus in Stethophyma localisation is more complete in some bivalents than others and in some individuals than others (White 1936). Again in Bryodema, nine of the eleven acrocentric autosomal bivalents possess only one chiasma in the long arm; this forms adjacent to, or in the immediate neighbourhood of the centromere. But two chromosomes do not conform to this behaviour (White 1954). The smallest bivalent invariably forms a single distal chiasma, as too does the fifth bivalent, but in this case 64 of the 166 first meta-phases examined also had a proximal chiasma leading to the production of a ring bivalent. The same principle applies to plants too. Thus in Allium fistulosum there is an average of 1.7 non-localised chiasmata per cell.