Data from: Mismatch between pollen and pistil size causes asymmetric mechanical reproductive isolation across Phlox species

Characterizing the mechanisms of reproductive isolation (RI) between
lineages is key to determining how new species are formed and maintained.
In flowering plants, interactions between the reproductive organs of the
flower –the pollen and the pistil– serve as the last barrier to
reproduction before fertilization. As such, these pollen-pistil
interactions are both complex and important for determining a suitable
mate. Here, we test whether differences in style length (a part of the
pistil) generate a postmating prezygotic mechanical barrier between five
species of perennial Phlox wildflowers with geographically overlapping
distributions. We perform controlled pairwise reciprocal crosses between
three species with long styles and two species with short styles to assess
crossing success (seed set). We find that heterospecific seed set is
broadly reduced compared to conspecific cross success and reveal a
striking asymmetry in heterospecific crosses between species with
different style lengths. To determine the mechanism underlying this
asymmetric reproductive isolating barrier we assess pollen tube growth in
vivo and in vitro. We demonstrate that pollen tubes of short-styled
species do not grow long enough to reach the ovaries of long-styled
species. We find that short-styled species also have smaller pollen and
that both within and between species pollen diameter is highly correlated
with pollen tube length. Our results support the hypothesis that the small
pollen of short-styled species lacks resources to grow pollen tubes long
enough to access the ovaries of the long-styled species, resulting in an
asymmetrical, mechanical barrier to reproduction. Such reproductive
isolating mechanisms, combined with additional pollen-pistil
incompatibilities, may be particularly important for closely related
species in geographic proximity that share pollinators.