DEMONSTRATION OF CRYPTIC INCOMPATIBILITY IN DISTYLOUS AMSINCKIA DOUGLASIANA

Hand pollinations experiments with distylous Amsinckia douglasiana A. DC. (Boraginaceae) revealed that legitimate (intermorph) crosses produce more seed than self-pollinations and that self-pollinations yield more seed than illegitimate (intramorph) crosses. We tested the occurrence of cryptic incompatibility in both style-length morphs by pollinating them with a mixture of legitimate and illegitimate pollen, each homozygous for a different electrophoretically distinguishable allele of phosphoglucose isomerase. The success of intramorph pollen was greatly reduced in mixture; only about 4% of the offspring of both pins and thrums were sired by illegitimate pollen. One pin and one thrum had sufficiently high seed set that the results cannot be attributed to selective abortion of embryos. In those individuals, discrimination against illegitimate pollen must have occurred on the stigma or in the style. Based on our findings, it appears that illegitimate matings are rare in natural populations. The progeny of legitimate crosses was biased toward pins, which suggests some regulation of morph ratios independent of the style-length locus. Prezygotic discrimination against the thrum genotype or selective abortion of embryos with the thrum genotype are both possible, but selective mortality of seedlings with the thrum genotype cannot be ruled out for some families.