Abstract
The New World grasshopper Cornops aquaticum (Leptysminae: Acrididae) shows a geographical pattern for three Robert- sonian polymorphisms in its southernmost area of distribution in Argentina and Uruguay. The frequency and distribution of chias- mata were analysed in five Argentinian populations. This study reveals a strong redistribution of chiasmata in fusion carriers, with a reduction in proximal and increase of distal chiasma frequency in fusion bivalents and trivalents, when all three karyotypes were compared. However, when only fusion bivalents and trivalents were compared, chiasma frequency was significantly higher in the former than in the latter. This higher chiasma frequency in fusion bivalents is due to an increase in proximal chiasma frequency. It is argued that the reduction in proximal chiasma frequency (relative to unfused bivalents) in fusion bivalents may be due to interfer- ence across the centromere. Proximal chiasma reduction in trivalents may be attributed either to a physical effect of structural hetero- zygosity or to an adaptation to the polymorphic condition. Therefore the differences in the distribution of chiasmata in trivalents and Robertsonian bivalents have different causes.
Highlights
Robertsonian rearrangements, as well as inversions and whole-arm reciprocal translocations (WARTs), are chromosome rearrangements that may cause a reduction in fertility in structural heterozygotes due to meiotic irregularities, to a lesser extent than interchanges (Hewitt, 1979)
If the two types of fusion carriers are compared, i.e. fusion homozygotes and heterozygotes, by means of Fisher’s least significant difference (Fisher LSD) contrasts, the proximal chiasma frequency decreases in trivalents and Robertsonian bivalents relative to standard bivalents (P < 0.00001 in both cases), but there are significant differences between fusion bivalents and trivalents (P = 0.0277) (Table 2, Fig. 2); with the trivalents having a significantly lower frequency of proximal chiasmata than fused bivalents
Whenever Robertsonian rearrangements are found in a polymorphic state, the chiasma frequency in trivalents is remarkably reduced, in particular the proximal chiasma frequency (Colombo, 1989, 1990, 1993; Bidau, 1990; Bidau & Martí, 2005; Hewitt & Schroeter, 1968)
Summary
Robertsonian rearrangements, as well as inversions and whole-arm reciprocal translocations (WARTs), are chromosome rearrangements that may cause a reduction in fertility in structural heterozygotes due to meiotic irregularities, to a lesser extent than interchanges (Hewitt, 1979). In the case of Robertsonian translocations the most frequent irregularity in heterozygotes consists of linear orientation in metaphase I, leading to imbalanced gametes This is most frequent in Robertsonian sub-metacentrics with highly unequal arm length and high chiasma frequency (Baker & Bickham, 1986; Bidau, 1991; Searle, 1993). This means that acro-telocentric pairs with roughly the same length and low chiasma frequency are preadapted to undergo a Robertsonian change that might lead to a polymorphism, especially if chiasmata are localized distally (Bidau & Mirol, 1988). In some cases the Robertsonian change leads to a reduction in chiasma frequency and distal localization of chiasmata in the rearranged chromosomes, amidst chromosomes on which the frequency and distribution of chiasmata are unrestricted (Colombo, 1993; Bidau, 1990)
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