Insights into the past, present and future of a narrow hybrid zone in the <i>Impatiens</i> of the Western Ghats, India

Hybrid zones are dynamic systems shaped by multidimensional ecological and evolutionary forces. When a hybrid zone is not entirely regulated by the environment or hybrid fitness, tracing the movement of populations and traits can provide further insights into the interplay between selective processes. Hybridization between two diverging plant lineages, Impatiens rosea and Impatiens balsamina, has formed a narrow hybrid zone in the Western Ghats of India that is not entirely environment-dependent nor regulated by hybrid fitness. To understand its maintenance, we studied the genetic structure of the hybrid zone using SNP markers. We assessed the role of inbreeding in maintaining population boundaries and investigated introgression, genetic composition, clinal patterns and the demographic history of the hybrid zone. The demographic analysis confirmed the origin of the hybrid zone in secondary contact. We found lower levels of heterozygosity and higher inbreeding in the hybrid zone than in parental populations. The hybrid zone is composed mainly of F2 or advanced-generation hybrids, suggesting minimal active hybridization, while inbred advanced-generation hybrids dominate the hybrid zone. Hybrids show asymmetric admixture with higher contribution from I. balsamina than I. rosea. Introgression is unidirectional from I. balsamina to I. rosea. I. balsamina is likely invading into I. rosea through a narrowly maintained hybrid zone. The narrow I. rosea-balsamina hybrid zone is a dynamic system that persists through inbreeding amongst hybrids and is regulated by a complex interplay of multiple ecological and evolutionary factors, offering insights into how such zones regulate landscape-level population dynamics.