# Ecological Isolation Maintains the Species Boundaries Between Two Sympatric Cycas From Southwest China

## TL;DR

Two cycad species in China maintain distinct identities without hybridizing, thanks to differences in timing, pollinators, and habitats.

## Contribution

Demonstrates ecological and reproductive isolation mechanisms in sympatric cycads without geographic barriers.

## Key findings

- Sympatric Cycas species show no genetic admixture despite shared habitats.
- Reproductive isolation is driven by divergent coning times and pollinator specificity.
- Ecological niche differentiation includes habitat and soil pH preferences.

## Abstract

Over long‐term evolutionary processes, sympatric affinities may develop reproductive isolation mechanisms, such as temporal isolation and ecological divergence, to maintain species independence. However, due to lacking strict geographic isolation barriers, sympatrically distributed closely related species may experience interspecific gene flow and genetic introgression, which can blur species boundaries. Here we focus on two sympatrically distributed Cycas species along the Lancang (Mekong) River in three populations from Southwest China,
Cycas pectinata
and
C. simplicipinna
, to investigate the extent of genetic introgression between them and how they maintain species boundaries. Using 16 microsatellite markers, we first genotyped and assessed the introgression patterns between the two species and their seedlings in each population. We further compared their geographical and ecological divergence, including the fine‐scale spatial distribution, habitats, reproductive phenology, and pollinators, based on a systematic field survey across its entire range in China. We found that sympatric populations of
C. pectinata
and
C. simplicipinna
, along with their seedlings, showed no genetic admixture. Further evidence supports that the two species exhibited significant variations in habitat indicators such as slope position and soil pH. Additionally, significant differences were observed in pollinator communities and coning behavior. These findings indicate that there is no hybridization between
C. pectinata
and
C. simplicipinna
under natural conditions. Instead, they maintain species boundaries primarily through reproductive isolation driven by divergent coning times and pollinator specificity, coupled with niche differentiation. This study not only provides a representative case for understanding mechanisms of plant species boundary maintenance but also offers critical theoretical support for the reintroduction and conservation of cycads.

The study reveals that sympatric Cycas species (
C. pectinata
and
C. simplicipinna
) maintain strict species boundaries without hybridization, driven by reproductive isolation mechanisms (divergent coning times and pollinator specificity) and ecological niche differentiation (habitat preferences, soil pH differences). These findings highlight how plants sustain species independence in the absence of geographic barriers, offering critical insights for cycad conservation and understanding evolutionary isolation mechanisms.