The ecology of hybrid zone formation and maintenance in Impatiens

Premise of research. Narrow hybrid zones are maintained by the counteracting forces of selection and gene flow. Understanding the mechanisms of formation and maintenance of narrow hybrid zones helps elucidate the evolutionary forces shaping reproductive biology, distribution, and diversification. We studied the ecology of the hybrid zone of two Impatiens species, Impatiens rosea and Impatiens balsamina, and the causes of the hybrid zone’s restriction in space. Methodology. We characterized floral morphometry for the identification of hybrids and parents. To understand the selection on floral traits, we modeled and compared geographic clines of floral traits. We assessed the effect of environmental niche, local adaptation, reproductive niche, and hybrid fitness on maintaining the structure of the hybrid zone. We studied two hybridization sites, one with an altitudinal gradient (Mulshi) and one without (Dapoli), to test the importance of environmental gradients. Pivotal results. We found that pollination-related floral traits, the length of the nectar spur and the size and shape of the wing petal, differ strongly between the parental populations; that these traits are intermediate in hybrids; and that they form the steepest coincident clines across the hybrid zone. Abiotic environmental niche and hybrid fitness do not regulate the hybrid zone structure. Parental species and hybrids show overlapping pollinators but differ in visitation frequencies. Local adaptation for reproductive success influences hybridization only in the site with an altitudinal gradient. Conclusions. The hybrid zone structure and its influencing factors differ across hybridization sites with and without altitudinal gradient. Neither the tension zone model nor the environment-dependent selection model completely explain this hybrid zone. Pollinator-mediated selection on floral traits is important in maintaining the narrow hybrid zone structure. Ecological factors primarily responsible for lineage divergence are also most likely to govern the dynamics of hybridization of the lineages.