The inter-relationship between heterochromatin distribution and chiasma distribution

Abstract

The distribution of chiasmata and their relationship to the presence of fixed and polymorphic heterochromatic segments is described in seven grasshopper species. In six of these, Cryptobothrus chrvsophorus, Trimerotropis bilobata, Calliptamus wattenwvlianus, Arc),ptera fusca, Pezotettix giorni and Acrotylus insubricus, the presence of terminally located polymorphic heterochromatic segments leads to a radical redistribution of chiasmata away from the segments to more proximal sites. Polymorphisms for proximal heterochromatic segments exist in the first three of these species and they lead to a predominance of terminally associated homologues at male meiosis. In Oxyajaponica where both polymorphic and fixed blocks are present, the polymorphic blocks have a similar pronounced effect on chiasma distribution, whereas the fixed blocks have no such effect.

Chiasma redistribution in presence of supernumerary chromosome segments in grasshoppers: dependence on the size of the extra segment

“…binotatus is that all extra segments do affect chiasma distribution on the bivalent carrying them, independently of their nature, size, chromosomal location and C-banding reaction, chiasmata being preferentially located in the furthest chromosome regions from the extra segment. This is consistent with former observations on heterogeneous sample of extra segments (Camacho et a!., 1984;Navas-Castillo et a!., 1985;John and King, 1985;de la Torre et a!., 1986). …”

“…Thus chiasmata are formed preferentially in the furthest chromosome regions from the extra segment, a fact also observed by John and King (1985) and de la Torre et a!. (1986).…”

The supernumerary segment systems of Rumex acetosa

“…These supernumerary segment systems are particularly prevalent in Orthoptera and usually involve blocks of heterochromatin (John, 1981) although some euchromatic segments have been found (Camacho et a!., 1984). No effects of these segments on the external phenotype have been demonstrated but a close relationship exists between the presence or absence of the segments and the distribution of chiasmata (John and King, 1985). It has been argued that this redistribution of chiasmata is of adaptive significance (John, 1981).…”

Reproductive anatomy and gametogenesis inShipleya inermis(Cestoda: Dioecocestidae)

“…We judged that the disparate arms were non-pairing, achiasmatic due to non-homology or to recombinational restriction associated with the presence of heterochromatin. The effects of heterochromatic blocks on chiasma-formation have been described (Southern, 1970;John and King, 1985;Nagl, 1985;Charlesworth et al, 1986). That a non-pairing portion remained discrete suggests that females of S. inermis produce gametes of two types, one euchromatic and one bearing a substantial amount of heterochromatin that may act as sexual factor.…”