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Age specific fitness traits were analyzed in two species of Drosophila, pseudoobscura and persimillis, to determine how hybrid fitness changes from parental taxa.

Among flowering plants, females often have little control over the genetic relatedness of pollen deposited on stigmas. Thus, postpollination processes are of primary importance for mate discrimination. The ability to screen and select among male gametes during pollen tube growth within the female tissues of the stigma, style, and ovary is critical to the process of mate choice and reproductive isolation. However, direct evidence of the mechanistic/developmental processes associated with mate choice in flowering plants is sparse. We studied the contribution of postpollination, prefertilization processes to reproductive isolation in two wind-pollinated species of birch (Betula) that commonly hybridize in nature. We exploited the 3-fold difference in ploidy level between these two species to determine the paternity of individual pollen tubes growing within female reproductive tissues. We then tracked their developmental fate in conspecific, heterospecific, and mixed-species crosses. This design allowed a direct comparison of developmental processes responsible for reproductive isolation in both single-species and mixed-species pollinations. Our results have important implications for the population genetic outcome of hybridization dynamics in natural populations. Paternity analysis of progeny from mixed-species pollinations revealed that conspecific pollen tubes sired more than 98% of seedlings. Biased siring success was not the result of differential embryo abortion. We detected strong, early postpollination barriers such as pollen tube incompatibility, slower pollen tube growth, and delayed generative cell mitosis. Conspecific fertilization precedence was mediated by favorable or unfavorable male-female interactions, but there was no evidence for antagonistic male-male interactions.

Temporal variation in cross pollination using heterospecific and conspecific pollen was used in the investigation of the relative contribution of pollen tube growth and attrition in reproductive isolation in two naturally hybridizing species of irises, Iris fulva and I. brevicaulis. Results indicate that pollen tube growth is an isolating mechanism in sexual reproduction but it has a more dominant contribution in I. fulva.

A central issue in studies of hybridization is the role of the environment in structuring hybrid zones. Two conceptual frameworks have been developed to address this issue. One group of [...]

The frequency of fruit formation, hybridization and interspecific pollen competition was studied in crossing experiments of Iris fulva and Iris hexagona. The frequency of fruit formation of crossed 600 I. fulva and 200 I. hexagona plants was 0.34 for I. fulva and 0.49 for I. hexagona with seven out of 710 seeds being hybrids. Differential fertilization by intraspecific pollen as a reproductive barrier was done by controlled crossing of 1:1 mixture of pollen from I. fulva and I. hexagona. No hybrids were produced from the 201 progeny suggesting that hybrid production is a relatively rare event.

This review focuses on two related outcomes of hybridization sensu lato (i.e. findings from both natural and experimental hybrids are discussed): (1) the production of hybrid genotypes with various fitnesses, and (2) the origin/transfer of adaptations in the Louisiana Iris species complex. Since effects on fitness, and the origin or transfer of adaptations, are of fundamental evolutionary importance, the examples discussed in this review reflect some of the most significant phenomena deriving from the transfer of genetic material via natural hybridization. [ABSTRACT FROM AUTHOR]

Examines the sources of genetic structure in the woody perennial Betula occidentalis in the United States. Identification of the species latitudinal range; Detection of high level gene flow using hierarchical analysis; Adaptive significance of polyploidy and hybridity of the species.