The B-chromosome constitution of a seed sample of Allium schoenoprasum collected from a sub-population at the River Wye, Powys, S. Wales in July 1982 has been compared with that of a seedling sample collected from the same sub-population in September 1982. The seedling sample contained significantly more B-containing plants than the seed sample (64.1 per cent vs. 55•2 per cent), and the mean number of B-chromosomes in B-containing plants was also significantly increased in the seedlings (244 vs. 2.23). This implies that there is differential selection in favour of B-containing seedlings and seedlings with increased numbers of B-chromosomes during the initial stages of the life cycle when levels of mortality of A. schoenoprasum in this riverside habitat are high.Non-standard types of B-chromosome accounted for a smaller proportion of the B-chromosomes in the seedlings compared with that in the seeds (9.7 per cent vs. 152 per cent), and there was a marked reduction in the frequency of plants carrying only non-standard B-types amongst the seedlings (6.2 per cent vs. 125 per cent).

The effects of B chromosomes in Allium schoenoprasum on germination rate and on the ability of seeds to germinate under drought conditions have been investigated. When water was freely available for germination, there was an increased frequency of B-containing individuals amongst the first 10 per cent of seeds to germinate compared with the middle germinating group (64 cf. 56 per cent). Two drought regimes were employed: in one ('droplet') seeds were provided with a limited amount of water, and in the other ('PEG') the availability of water was limited osmotically. There was a significantly increased frequency of B-containing individuals amongst the 5 per cent of seeds that were able to germinate in both the droplet sample (79 per cent) and the PEG sample (65 per cent) compared with the control sample (56 per cent). Although the early and drought germinating groups had higher frequencies of B-containing seedlings, the individual seedlings did not carry a significantly increased number of B chromosomes. The increases in the frequencies of B-containing seedlings in the early and drought germinating groups were comparable to or greater than that observed previously in the wild between the seed and seedling stages, suggesting that the increased survival of B-containing individuals in natural populations could be due to the effects of Bs on germination behaviour. There was no relationship between the presence of B chromosomes and seed size, seed density or seed shape, implying that differences in these seed characteristics do not provide an explanation of the effects of B chromosomes on germination.

SUMMARYThe concept of the 'nucleotype' embraces aspects of the nuclear DNA that affect the phenotype other than through the coding sequences. In this context, we consider one particular situation, namely DNA variation due to the presence of supernumerary B chromosomes (Bs); these Bs, which are additional to the basic complement of A chromosomes, generate a spectrum of DNA amounts, and thus nucleotypes, within many species. We consider the physiological consequences of carrying extra chromosomes that appear by and large to lack coding sequences, and focus on two species with contrasting case histories: rye (Secale cereale L.) and chives (Allium schoenoprasum L.), which illustrate 'selfish' and 'adaptive' explanations of the persistence of B chromosomes.

The level of inbreeding depression has been measured at four independent stages of the life cycle (parental seed fecundity, seed viability, seedling survival and seedling growth. rate) using wild-collected parental plants from seven natural populations of Allium schoenoprasum. At each stage, progenies produced by self-fertilisation were significantly less fit than progenies produced by cross-fertilisation, and the overall inbreeding depression was severe (0.718). This suggests that a high level of cross-fertilisation usually occurs in natural populations, despite the fact that A. schoenoprasum is a clonal, self-compatible, hermaphrodite plant, with apparently ample opportunities for geitonogamous selfing.Using a white flower colour marker, the minimum outcrossing rate in a natural population has been estimated as 091. However, this is almost certainly an over-estimate due to the unequal viabilities of cross and self zygotes. An adjusted estimate of ta,,, = 080 is derived which takes account of the lower survival rate of self zygotes.

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