The Reproductive System of Amsinckia grandiflora, a Distylous Species

The flowers of Amsinckia grandiflora are distylous and the pollen is dimorphic in size. The single knozwn population of this annual species contained a 1: 1 representation of the two morphs during two seasons, but a 2: 1 pin: thrum ratio in a third season. The chief insect visitor to the flowers is Anthophora edwardsii, a bee that preferentially removes thrum pollen. Pin stigmas receive more pollen grains than thrum stigmas and also a greater proportion of illegitimate pollen than thrums. Seasonal fluctuations in the representation of the two morphs have no striking influence on pollen-flow patterns. Two sympatric, aggressive homostylous weeds of the same genus produce more seeds per flower than does A. grandiflora. It is possible that the greater reproductive success of these zweedy species has allowed them to displace A. grandiflora over much of the latter's former range. Amsinckia grandiflora Kleeberger (Boraginaceae) is an annual herb now restricted to a single, small population in San Joaquin County, California. The flowers are distylous, and the two floral forms differ conspicuously in the position of the anthers and the length of the style (Fig. 1-4). The floral morphology and highly restricted distribution of this species con- trast sharply with those of a number of other members of the genus, such as A. tessellata Gray and A. intermedia Fischer c Amsinckia is of particular interest because of the occurrence within it of distyly and an array of derived homostylous systems. In view of the con- trasts between the floral heteromorplhism and restricted distribution of A. grandiflora, a primitive species of the genus (Ray & Chisaki, 1957b), and those of its more successful homostylous congeners, I have studied the reproductive system of this species. This paper presents the results of this study and suggests that the present restricted distribution of the species is perhaps associated with its archaic and relatively inefficient reproductive system.