CORRELATION BETWEEN THE BREEDING SYSTEM AND RECOMBINATION INDEX IN FIVE SPECIES OF SENECIO

Abstract

SUMMARY The species Senecio aetnensis Jan., S. joppensis Dinsm., S. squalidus L., S. viscosus L. and S. vulgaris L. have been compared for automatic selfing ability, capitulum conspicuousness, pollen yield, chromosome number and chiasma frequency. It has been established that the species S. aetnensis, S. joppensis and S. squalidus which have low automatic selfing ability have large, showy capitula, high pollen output and low recombination indices. The species S. viscosus and S. vulgaris which have a high selfing ability have relatively inconspicuous capitula, low pollen yields and a high recombination index. The differences in recombination index values are mainly due to the fact that the selfing species are polyploids but it may be significant that the highest average chiasma frequencies per bivalent were found in some S. vulgaris plants. The adaptive significance of a high recombination index in inbreeding taxa is discussed and it is suggested that a high chiasma frequency and/or polyploidy may have a preadaptive value in facilitating the change from outbreeding to inbreeding in some species.

Inhibition of self pollen germination in Senecio vulgaris L.

“…Hybridisation with S. squalidus is thought to be responsible for the origin of the radiate allele in S. vulgaris (Ingram, 1977;Ingram et a!., 1980). Incompatibility in S. squalidus is, however, a feature of both ray and disc florets (Gibbs et a!., 1975), which prompts the question: why should a self-incompatibility dimorphism in S. vulgaris be restricted only to the ray florets? Although unusual, self-incompatible species such as purple loosestrife, Lythrum salicaria L. (Darwin, 1877).…”

“…If no such system existed, all self pollen would immediately germinate on the stigma. In S. vulgaris, which produces about 200 pollen grains per floret (Gibbs et a!., 1975), the stigma might be mechanically damaged if this happened. If all self pollen grains did germinate, outbreeding might be reduced because (a) the pollen is no longer free for transport to other plants and (b) the stigmatic surface is blocked to incoming pollen.…”

“…The species used to be considered highly inbred, with reproduction verging on complete autogamy (Gibbs et a!., 1975), but in experimental plots Campbell and Abbott (1976) found an average outcrossing rate of 224 per cent in nonradiate lines. Furthermore, Abbott (1982, 1984a), using the ray floret locus as a marker, showed that in populations polymorphic for capitulum type, the radiate morph had a greater "female" outcrossing frequency than the nonradiate morph.…”

Outcrossing frequencies within- and between-capitulum morphs in groundsel Senecio vulgaris L.

“…This was reported to be the case by Gibbs et a!., (1975), but Warren (1987) found frequent, and often site-specific, differences between morphs. A further assumption is that both greenand black-bract capitula are equally attractive to insects.…”

“…The consensus was that it indulged in only small amounts of outcrossing. Gibbs et al (1975) also considered it to be a highly inbred species, to the point of autogamy. In experimental plots, however, levels as high as 22 per cent female outcrossing have been recorded in non-radiate lines (Campbell and Abbott, 1976).…”