EXPERIMENTAL HYBRIDIZATION, CHROMOSOMAL DIVERSITY, AND PHYLOGENY WITHIN GAURA SECTION STIPOGAURA (ONAGRACEAE)

Abstract

Experimental hybridization data generally agree with patterns of morphological, ecogeographic, and chromosomal variation in support of current taxonomic hypotheses for Gaura section Stipogaura. Average rates of capsule‐set for intersectional crosses are significantly lower than for interspecific crosses within Stipogaura (14% vs. 63% respectively, P < 0.001). Post‐mating reproductive isolation within the section is not well‐developed, as indicated by comparable averages of cross‐success for interspecific (19%) and intraspecific (17%) crosses. Within Stipogaura, lowered rates of capsule‐set (for G. filipes crosses) and germination (seed from G. calcicola, G. filipes, and G. sinuata crosses) segregate the five species into four fertility groups. Gaura calcicola (n = 7) and G. sinuata (n = 14) differ by lower pollen staining in the hybrids of G. sinuata, a result of odd‐ploid hybridity. The order of average, interspecific cross‐success is G. mckelveyae (35%), G. villosa (25%), G. calcicola (14%), G. sinuata (10%), and G. filipes (2%). Patterns of interspecific fertility do not precisely parallel phylogenetic hypotheses based upon morphology. Three diploid taxa that closely resemble one another in growth habit (G. mckelveyae, G. calcicola, G. filipes) differ widely in rates of crossing success. Gaura villosa is relatively specialized morphologically but retains a high degree of interspecific fertility. Gaura sinuata is also specialized, but readily produces hybrids in crosses with three of the diploids.