Hybridisation in a wild population of eleocharis palustris

Abstract

1. Twenty-nine inflorescences from plants of Eleocharis palustris in a small area of Port Meadow, Oxford, were found to include four types. Two of these were the recognised sub-species E. p. vulgaris (2n=38) and E. p. microcarpa (2n=16). 2. The former was represented by seven inflorescences one of which was abnormal showing delayed or non-disjunction of the chromosomes at PGM I in approximately 8% of the cells. 3. The latter was represented by ten inflorescence two of which were abnormal showing a failure in 40% of the cells of the nuclear differentiation which normally precedes PGM. 4. In four of the other inflorescences the chromosome number at PGM I ranged from 11 to 18 with a modal value of 14. Plants with 2n=27 chromosomes in the root tips have also been found. We consider these to be F1 hybrids between the two subspecies. 5. In the remaining seven inflorescences, the chromosome number at PGM I ranged from 16 to 21 with a modal value of 18/19. These may be plants of E. p. vulgaris with an abnormal meiosis or hybrid derivatives. 6. The survival of the hybrid material in this population is attributed to the fact that sexual fertility is not at a premium. The meadow has been subjected to continuous grazing for centuries and under these curcumstances survival depends on vegetative vigour and propagation. Presumably the presence of fertility-affecting abnormalities in the parental types can be explained on the same basis as indeed, may the occurrence of hybridisation. 7. Numerical chromosome variation in the genus appears to be due to a variety of causes. a) Aneuploidy. α) Non-disjunction at meiosis (especially in hybrids or hybrid derivatives) or at pollen grain mitosis. β) Fragmentation, perhaps following inversion loop crossing over in structural hybrids—a source of variation which is excluded in most groups by the localisation of centromeres. b) Polyploidy. α) Restitution at meiosis (diploid self-fertilisation). β) Restitution at pollen grain mitosis (formation of homozygous diploid gametes).