Genomic coalescence in a population of Laxmannia sessiliflora (Angiospermae, Anthericaceae): an association of lethal polymorphism, self-pollination and chromosome number reduction

A population of Laxmannia R. Br. (Angiospermae, Anthericaceae) near Collie, Western Australia, combines the taxonomically significant sessile inflores cences of L. sessiliflora Dcne. (n = 4) and the derived breeding system of L. ramosa Lindl. (n = 4). It exhibits a polymorphism for seed-aborting leth al equivalents, significant levels of self-pollination and a chromosome pol ymorphism in which a haploid genome with n = 3 is most frequent. Allozyme a nalysis indicates that the population is either of hybrid origin or one tha t has uniquely diverged from a phylogenetic link between the two species. T he population is considered to represent a natural demonstration of the phe nomenon of genomic coalescence as modelled by James (1992, Heredity, 68, 44 9-456) in which devices which reduce the number of independently segregatin g supergenes heterozygous for recessive lethals are elevated to high freque ncies by inbreeding. The population also suggests a mechanism whereby dyspl oid chromosome number reduction may be promoted by natural selection in nat ural population systems.