Reproductive character displacement in Phlox.

Abstract

Plants of the red- and pink-flowered Phlox drummondii were introduced into a natural population of P. cuspidata. Their seeds were progeny-tested, and the percentage hybrid seed determined for each variant: 38% of the seed from the pink variant were hybrid compared to 13% of the red. The difference in hybrid production is best explained by corolla color. The self- and cross-compatibility of 9 sympatric and 9 allopatric populations were studied in the greenhouse. On the average, sympatric populations are more self-compatible than allopatric populations. Pollen germination is 13.7% vs. 7.3%. The self-compatibility differential was accompanied by a self seed-set differential. On average, sympatric populations are more cross-compatible with P. cuspidata than are allopatric populations. Germination of P. cuspidata pollen on sympatric P. drummondii stigmas was 13.3% vs. 9.8% on allopatric stigmas, but the difference is not statistically significant. The presence of P. cuspidata has promoted reproductive character divergence in P. drummondii. The shift in corolla color and the increase in self-compatibility reduce the potential for gametic wastage and interspecific hybridization. -Author