B chromosomes in Rattus fuscipes I. Mitotic and meiotic chromosomes and the effects of B chromosomes on chiasma frequency

Abstract

SUMMARYThe small heterochromatic B chromosomes of Rattus fuscipes assimilis are similar to those described by others in the related species Rattus rattus. They are apparently stable in somatic tissues, but in male meiosis they are liable to non-disjunction. The B chromosomes in R. f assimilis are associated with an increase in the mean cell chiasma frequency, at least in males.

B chromosomes in populations of mammals

“…Moreover, Thomson et al (1984) observed that the B chromosome in R. fuscipes from Australia is morphologically very similar to that of R. rattus from Japan (Yosida, 1977). However, besides origin from a common source, it is also possible that one chromosome from the karyotype particularly prone to polysomy could be a repeated source of new Bs.…”

B chromosomes in Rattus fuscipes II. The transmission of B chromosomes to offspring and population studies: support for the “parasitic” model

“…In theory B chromosomes could affect dispersal behaviours directly by the action of genes located on the B chromosomes themselves, either autonomously or via effects on genes on the A chromosomes. The C band-positive nature of the B chromosome (Yosida and Sagai, 1973;Thomson et aL, 1984), however, suggests that much of it is constitutively heterochromatic and thus probably carries relatively few active genes, although this requires confirmation.…”

“…Such random distribution would be achieved if (a) Bs did not pair at first division and moved randomly to either pole (evidence from R. fuscipes male meiosis suggests that is the case; Thomson, 1984).…”

“…Rats were released at the point of capture after being individually marked by the toe clipping method described by Flowerdew (1976). From observations of mitosis and male meiosis in R. fuscipes with B chromosomes (Thomson et a!, 1984) it is clear that there is little or no non-disjunction in premeiotic mitoses. This being the case, it is possible to gain some idea of the behaviour of B chromosomes during meiosis by observing the distribution of B chromosome classes amongst the progeny of directed crosses (tables 1-5).…”

Mitotic and meiotic behaviour of B-chromosomes ofGuizotia scabra(Vis.) Chiov. ssp.scabra(Compositae)

“…Some authors consider Bs as «parasitic» (OsTERGREN 1945;NuR 1977) or «selfish» (MATTEWS andJoNES 1982, 1983) with no known function other than that of their own perpetuation and maintenance. The Bs of some species are known to affec~ the frequency and distribution of chiasmata and consequently the phenotypic variation (THOMSON et al 1984;PARKER et al 1990). HIREMATH and MuRTHY (1986) reported the occurrence of a single Bchromosome of two morphological types, each in one population of Guizotia scabra (Vis.)…”