The consequences of monoecy and dioecy in congeneric species, and their implications for the evolution of dioecy in the genus Viscum

Abstract

Two species of Viscum, the dioecious Viscum capense ssp capense and the monoecious Viscum rotundifolium , were compared in terms of their ecology, repioductive biology and genetics in order to determine some potential selective advantages of dioecy in this genus Data was collected in such a way thiat several of the current hypotheses for the evolution of dioecy could be tested Selection for outcrossing; disruptive selection or decreased infraspecific competition; pollinator or fi ugivore attraction to tussive pollen or fruit crops and escape from seed predation were among the hyp?the~es investigated. The population structure of Viscum capense showed no evidence for disruptive selection, both in terms of host choice, and associations between plants of different sexes. Viscum rotundifolium was found to be more highly clumped than Viscum capense, which is an advantage in terms of disnerser attraction, but a disadvantage in terms of seed predation. A closer examination of the results, however, showed that the distribution of seed-bearing plants in both species was not dependant on the breeding system. Pollinator observations eliminateo the hypothesis that large polien crops would be advantageous in terms of pollinator attraction, since the pollinators were found to visit male flowers foi neotar, and not pollen. The genetic results showed that the dioecious Viscum capense and the monoecious Viscum rotundifolium did not differ in levels of genetic heterozygosity, and thus, it was assumed, that dioecy did not evolve in response to ^election * or outcrossing in this genus. These res jits also revealed a number of loci in both species which were fixed for heterozygosity, and some possible explanations for the mechanism by which these were maintained are put forward. No overwhelming selective advantage of dioecy could be determined in this case and it was hypothesized that dioecy could have been fixed in the population by chromosomal translocations which also facilitated rapid speciation, thus enabling the gene combinations for dioecy to escape elimination by selection within the original gene pool