Heterostyly: speciation within a species

Almost all organisms in nature show nonrandom mating to different degrees. Two extreme results of nonrandom mating are speciation and sexual differentiation. Heterostyly is a form of sexual differentiation considered to have evolved to resolve conflicts between male and female functions of hermaphrodite flowers. Our study examines necessary and sufficient conditions for establishment of heterostyly using a configuration individual-based model. Previous models assume invasion of a mutant phenotype into a population with monomorphic wild phenotype. In contrast, our model demonstrates that heterostyly can be established from a population with continuous phenotypic variation, a proposition that requires simpler assumptions than the previous hypotheses. Results of our simulation show that genetic linkage between stigma and anther heights is essential for establishment of heterostyly. Dominance effects on the genes for stamen or stigma heights are not necessary, but they promote evolution of heterostyly. Probability of evolution of heterostyly also depends on the functional relationship between stigma–anther distance and strength of sexual interference, and the distance and probability of pollen deposition success. Parallelity and difference between speciation and sexual differentiation are also discussed

Microsporogenesis in male-sterile and hermaphroditic plants of nine gynodioecious taxa of Hawaiian bidens (Asteraceae)

Microsporogenesis was investigated in hermaphroditic and male-sterile plants in nine gynodioecious taxa of Hawaiian Bidens. Normal microsporogenesis in hermaphrodites and the onset of abortion in male steriles were similar in all taxa and in a hybrid between two gynodioecious species. The early abnormal vacuolation of tapetal cells is the first visible evidence leading to premeiotic abortion of microsporogenesis in male steriles. The sporogenous cells disintegrate rapidly after the vacuolation of the tapetum, resulting in a shrunken, indehiscent anther which is composed of only the epidermal layer with some remnant cells of the endothecium and the connective at anthesis. In hermaphrodites, the tapetal cklls remain dense and undergo karyokinesis to become binucleate during meiosis I. The tapetum becomes plasmodia1 after microspores are released from tetrads and gradually disappears during pollen formation. The genetic factor($ which cause the abortion act with remarkable precision and consistency in all taxa investigated. This suggests that gynodioecy in all Hawaiian Bidens is homologous and the establishment of male sterility in Hawaiian Bidens occurred only once. The spread of the genetic male-sterile factor(s) may be the result of adaptive radiation of the original gynodioecious species or natural interspecific hybridization.link_to_subscribed_fulltex

Mixed mating systems in Hawaiian Bidens (Asteraceae)

Protandry and male sterility promote outcrossing, while self-compatibility and geitonogamy contribute to inbreeding. The combination of these floral mechanisms results in a mixed mating system in all species studied. Outcrossing rates of 15 populations of the 11 species ranged from 0.43-0.88, averaging 0.65. Apparent selfing rates of females ranged from 0-0.25 in 7 gynodioecious populations, suggesting that there is variation in the level of biparental inbreeding among populations. Presence of females increased the level of outcrossing by an average of 9% in gynodioecious populations. Efficiency of gynodioecy as an outcrossing mechanism largely depends on the current outcrossing rate of hermaphrodites, frequency of females, and extent of genetic substructuring in populations. On average, autogamy contributed 4%, geitonogamy contributed 24%, and consanguineous mating contributed 15% to the realized selfing rate (43%) in the hermaphrodites of these species. -from Authorslink_to_subscribed_fulltex

Genetics of gynodioecy in Hawaiian Bidens (Asteraceae)

Inheritance of male sterility in all gynodioecious species of Hawaiian Bidens was investigated by experimental crosses conducted in greenhouses. Both male-sterile individuals and hermaphrodites were segregated in the progenies of openpollinated female and hermaphroditic plants growing in nature. The ratios (H:MS) of 1:0, 1:1, 3:1 were observed in the F1 generations; 3:1 and 15:1 in the F2 generations; and 1:1 and 3:1 in the backcrosses. Thirteen interspecific triple crosses and all quadruple crosses gave MS progenies, indicating a directly allelic relationship among male sterility genes in all gynodioecious species. Based on these results, A digenic-cytoplasmic model was proposed, and the evolution of gynodioecy in Hawaiian Bidens was discussed. © The Oenetical Society of Great Britain.link_to_subscribed_fulltex

Outcrossing rates and allozyme variation in rayed and rayless morphs of Bidens pilosa

Thirty-four isozyme loci were assayed in 1048 plants from three Hawaiian populations of the autogamous introduced weed Bidens pilosa. Total isozyme gene diversity was very low, 0-049. One population containing only rayless plants was completely monomorphic at all loci except PGI-3, and this locus was nearly fixed. In a large population polymorphic for rayed, rayless, and intermediate plants, two loci showed rare variants and PGI-3 was polymorphic. Allele frequencies at PGI-3 were not significantly different among the floral morphs, but outcrossing rates, measured by progeny tests using PGI-3 as a genetic marker, were significantly higher in the radiate morph (9 per cent) than in the rayless and intermediate morphs (5 per cent). Presumably radiate plants have higher outcrossing rates because they are more attractive to pollinators. © The Genetical Society of Great Britain.link_to_subscribed_fulltex