Gene flow inDubautia arboreaandD. ciliolata: the roles of ecology and isolation by distance in maintaining species boundaries despite ongoing hybridization

Abstract

The relative roles of gene flow and natural selection in maintaining species differentiation have been a subject of debate for some time. The traditional view is that gene flow constrains adaptive divergence and maintains species cohesiveness. Alternatively, ecological speciation posits that the reverse is true: that adaptive ecological differentiation constrains gene flow. In this study, we examine gene flow and population differentiation among populations of two species of the Hawaiian silversword alliance, Dubautia arborea and D. ciliolata. We compare divergence in putatively neutral microsatellite markers with divergence in leaf morphometric traits, which may be selectively important or physiologically linked to selectively important traits. Gene flow between populations was found to be significant in only one of the two species, D. arborea. Leaf morphometric differentiation between species was significant, though not among populations within species. No evidence of effective genetic introgression was observed between apparently 'pure' populations of these species. Gene flow as measured by microsatellites was not correlated with geographic distance between populations, but was correlated with the linear placement of the widest part of the leaf. Because these two species are interfertile, as demonstrated by the presence of active hybrid zone, the lack of genetic introgression and the maintenance of species boundaries may be associated with natural selection on differential habitat.

Little interspecific pollen transfer despite overlap in pollinators between sympatric Aeonium (Crassulaceae) species pairs

“…In general, most pollinators visit broader spectra of plants and only few interactions are highly specialised (Goulson, 2003;Levin and Anderson, 1970;Olesen et al, 2002;Waser et al, 1996;Waser, 1998). In accordance with our observations, Friar et al (2007) described the use of generalist pollinators for species of the Hawaiian Silverswords enabling gene flow between them. Despite an overlapping pollinator spectrum, the proportion of infraspecific pollen transfer (averaged 95%) always significantly exceeded the interspecific transfer (3%-21% for specific species pairs).…”

“…Adaptive radiations are defined by species evolving rapidly in response to ecological differences correlating with a burst of morphological and physiological diversity (Schluter, 2000). Given the high tempo of speciation and that the species are closely related, isolation barriers are rather weak or absent and hybridization is generally easy (Francisco-Ortega et al, 2000;Friar et al, 2007;Remington and Robichaux, 2007;Whitkus, 1998). Hence, ongoing interspecific gene flow and hybridization may have an important influence on adaptive radiations.…”

“…Hence, ongoing interspecific gene flow and hybridization may have an important influence on adaptive radiations. Several studies have tried to detect hybrid origin of species and gene flow within and between species and populations using diverse molecular genetic markers (Friar et al, 2007;Lawton-Rauh et al, 2007;Remington and Robichaux, 2007). However, given their genetic similarity the ability to detect ongoing gene flow between species via molecular markers may be limited, particularly in adaptive radiations (Friar et al, 2007).…”

Asymmetrical Gene Flow in a Hybrid Zone of Hawaiian Schiedea (Caryophyllaceae) Species with Contrasting Mating Systems

“…Actually, instances of asymmetric gene flow in hybrid zones are commonly documented in other natural systems [3], including Hawaiian taxa. For example, Friar et al [35] identified ongoing hybridization from Hawaiian Dubautia ciliolata to D. arborea leading to the formation of an active hybrid zone. However, there was little evidence of introgression into parental populations [35].…”

“…For example, Friar et al [35] identified ongoing hybridization from Hawaiian Dubautia ciliolata to D. arborea leading to the formation of an active hybrid zone. However, there was little evidence of introgression into parental populations [35]. Randell et al [41] also found asymmetrical hybridization between Rubus hawaiiensis and R. rosifolius on Maui based on the presence of shared cpDNA haplotypes between the hybrids and R. rosifolius but not R. hawaiiensis .…”