Increased floral divergence in sympatric monkeyflowers

Area of Science:

• Evolutionary Biology
• Speciation
• Plant Reproductive Biology

Background:

• Sister species are predicted to show greater reproductive trait divergence when sympatric compared to allopatric.
• This divergence is expected if mating system shifts, like self-fertilization, are more probable within the ancestor's outcrossing range.
• The monkeyflower genus, Mimulus, provides a model system to test these speciation hypotheses.

Purpose of the Study:

• To investigate divergence in reproductive traits between sympatric and allopatric sister species in Mimulus.
• To determine if shifts in mating systems, such as the evolution of self-pollination, correlate with geographic distribution (sympatry vs. allopatry).
• To explore potential mechanisms driving speciation in Mimulus, including reinforcement and parapatric speciation models.

Main Methods:

• Comparative analysis of flower size divergence between sympatric and allopatric Mimulus sister species.
• Assessment of floral morphology, specifically the relative position of anthers and stigmas, to infer pollination mechanisms (outcrossing vs. self-pollination).
• Evaluation of potential speciation mechanisms like reinforcement and stepping stone models.

Main Results:

• Sympatric Mimulus sister species exhibit significantly greater divergence in flower size compared to allopatric sister species.
• Sympatric sister species are more frequently characterized by one species having anthers that overtop stigmas, indicating a propensity for automatic self-pollination.
• These findings support the hypothesis that reproductive isolation evolves more rapidly in sympatry.

Conclusions:

• Geographic context (sympatry vs. allopatry) influences the rate of reproductive trait divergence during speciation.
• The evolution of automatic self-pollination may be a key factor contributing to reproductive isolation and speciation in sympatric Mimulus.
• Reinforcement and parapatric speciation models offer plausible explanations for the observed patterns of divergence.