Abstract
Asymmetrical gene flow, which has frequently been documented in naturally occurring hybrid zones, can result from various genetic and demographic factors. Understanding these factors is important for determining the ecological conditions that permitted hybridization and the evolutionary potential inherent in hybrids. Here, we characterized morphological, nuclear, and chloroplast variation in a putative hybrid zone between Schiedea menziesii and S. salicaria, endemic Hawaiian species with contrasting breeding systems. Schiedea menziesii is hermaphroditic with moderate selfing; S. salicaria is gynodioecious and wind-pollinated, with partially selfing hermaphrodites and largely outcrossed females. We tested three hypotheses: 1) putative hybrids were derived from natural crosses between S. menziesii and S. salicaria, 2) gene flow via pollen is unidirectional from S. salicaria to S. menziesii and 3) in the hybrid zone, traits associated with wind pollination would be favored as a result of pollen-swamping by S. salicaria. Schiedea menziesii and S. salicaria have distinct morphologies and chloroplast genomes but are less differentiated at the nuclear loci. Hybrids are most similar to S. menziesii at chloroplast loci, exhibit nuclear allele frequencies in common with both parental species, and resemble S. salicaria in pollen production and pollen size, traits important to wind pollination. Additionally, unlike S. menziesii, the hybrid zone contains many females, suggesting that the nuclear gene responsible for male sterility in S. salicaria has been transferred to hybrid plants. Continued selection of nuclear genes in the hybrid zone may result in a population that resembles S. salicaria, but retains chloroplast lineage(s) of S. menziesii.
Highlights
Analyses of hybrid zones have provided a wealth of information on factors that favor interspecific hybridization, the nature of preand post-zygotic barriers to hybridization, and the stability of hybrid zones [1]–[][3]
We found a high degree of similarity between the hybrids and S. salicaria in the traits associated with wind pollination, including greater pollen production, smaller pollen size, more flowers per inflorescence, and the details of inflorescence shape (Figs. 1, 2)
We have documented the presence of an interspecific hybrid zone between two species of Schiedea on West Maui and found evidence of unidirectional cytoplasmic gene flow contributing to the diversity of the current hybrid population
Summary
Analyses of hybrid zones have provided a wealth of information on factors that favor interspecific hybridization, the nature of preand post-zygotic barriers to hybridization, and the stability of hybrid zones [1]–[][3]. These studies provide insights into the role of hybridization in contemporary natural populations as well as past hybridization events. Contrasts between maternally-inherited chloroplast DNA and nuclear DNA inherited through both parents can provide confirmation of hybridization and evidence for the direction of gene flow [7]–[11]. Demographic factors may play an important role in determining the direction of introgression as Currat et al [3] found strong evidence of asymmetrical introgression in a wide variety of taxa where gene flow occurred from a local to an invading species
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