Variation in floral morphology and ploidy among populations of Collinsia parviflora and Collinsia grandiflora

Variation in floral form complicates species identification, and when variation in ploidy also occurs, taxonomic designations are significantly compromised. We studied morphological and ploidy variation in sister species Collinsia parviflora Lindl. (small-flowered form) and Collinsia grandiflora Douglas ex Lindl. (large-flowered form), sampling from California, where the species were previously described as diploids, to British Columbia, where morphologically variable populations were more recently designated tetraploid. We found continuous among-population variation in flower size and shape throughout our sampled range. Ploidy variation, estimated using a combination of chromosome counts and flow cytometry, was not associated with flower size. Diploid populations were rare, and both large-flowered and small-flowered diploids were identified. Tetraploid populations were common, geographically dispersed, and exhibited the full range of flower sizes. A few populations of small-flowered plants were putative hexaploids. When large- and small-flowered forms co-occurred, they were diploid and tetraploid, respectively, suggesting morphological differentiation from selection for reproductive isolation between co-occurring plants of different ploidy. Although diploid C. parviflora (small-flowered) and C. grandiflora (large-flowered) occur, the majority of our sample consisted of morphologically variable tetraploids, suggesting that the use of flower size to delineate species is inappropriate. Further research is needed to ensure a true description of species boundaries in this group.