REPRODUCTIVE ISOLATION BETWEEN SYMPATRIC RACES OF PEA APHIDS. I. GENE FLOW RESTRICTION AND HABITAT CHOICE.

Area of Science:

• Evolutionary Biology
• Population Genetics
• Behavioral Ecology

Background:

• Understanding barriers to gene flow is crucial for analyzing reproductive isolation between populations.
• Sympatric pea aphid (Acyrthosiphon pisum) populations on alfalfa and red clover are genetically divergent and locally adapted.

Purpose of the Study:

• To investigate habitat choice as a primary factor limiting gene exchange between sympatric pea aphid populations.
• To determine the extent of reproductive isolation and incipient speciation in these host-associated aphid races.

Main Methods:

• Hierarchical population structure analysis using Fst to estimate gene flow between host-associated populations.
• Field studies observing winged aphid colonists' settlement on new host plants.
• Laboratory experiments assessing host choice behavior in relation to demographic performance across different aphid genotypes.

Main Results:

• Gene exchange is highly restricted between pea aphid populations on different hosts compared to those on the same host.
• Aphids exhibit strong habitat fidelity, with a genetic correlation between host acceptance and performance on that host.
• Habitat choice results in assortative mating, contributing significantly to reproductive isolation and suggesting incipient speciation.

Conclusions:

• Habitat choice and subsequent assortative mating are major drivers of reproductive isolation in sympatric pea aphid populations.
• Observed genetic divergence exceeds levels explainable by dispersal alone, indicating additional barriers like post-zygotic selection or hybrid breakdown.