HETEROSTYLY, POPULATION COMPOSITION, AND POLLEN FLOW IN HEDYOTIS CAERULEA

Hedyotis caerulea is a perennial, spring-flowering herb native to eastern North America with distylous flowers that differ in a number of morphological and physiological traits. However, the pin and thrum morphs produce the same numbers of buds, flowers, and fruits per plant, although it is possible that differences in these may occur in some populations at certain times of the flowering period. The two morphs are self-incompatible and cross-compatible. Most populations contained an excess of pins over thrums (anisoplethy); less commonly pins and thrums were equally represented (isoplethy). Populations change from anisoplethy to isoplethy and in the reverse direction. The spatial distribution of pin and thrum flowers in populations was random in some populations but non-random in others. There is some indication that the two morphs in some populations have somewhat different flowering periods. Pollinators seem to be chiefly bombyliid flies and perhaps thrips, but insects were rarely observed visiting flowers. In some populations, the two morphs produced equal numbers of pollen grains per flower; in others they did not. The average pollen viability varied, but on the average there was a moderate level of pollen sterility. High numbers of pollen grains remained in dehisced anthers, probably as a result of low pollinator activity. However, between 5% and 9o of the pollen produced participated in pollination. Stigmas of most pin flowers received more pin pollen grains than thrum pollen grains; on stigmas of thrum flowers pin pollen grains outnumbered thrum pollen grains. Thus, compatible pollen flow from pins to thrums was greater than in the reverse direction. Anisoplethic and isoplethic populations had the same pollen flow patterns. A plantby-plant examination of stigmas indicated that many stigmas bore few or no pollen grains. Seed production of the two morphs was equal. Despite the inequities in pollen flow patterns, the widespread and occasionally weedy nature of H. caerulea suggests that its breeding system must be viewed as a successful one. PERHAPS as many as half the genera of angiosperms that are heterostylous are members of the large, mostly tropical family Rubiaceae (Bir Bahadur, 1968; Vuilleumier, 1967). Hedyotis caerulea (Linnaeus) Hooker, a member of subgenus Edrisia (=Houstonia in part; Lewis, 1962) is distylous, a feature noted as early as 1862 by Asa Gray (Gray, 1862, letter to Charles Darwin, Cambridge University Library; Ornduff, 1977). Commonly known as Bluets, H. caerulea is a delicate, spring-flowering herb that is widespread in North America, ranging from eastern Canada to the southeastern United States. This paper discusses aspects of its reproductive biology and population comI Received for publication 9 March 1979; revision accepted 6 July 1979. I am indebted to W. R. Anderson for carrying out the pollination program in 1962; S. G. Weller for assistance in devising sampling methods; a seminar group in the Department of Statistics at Berkeley for statistical assistance; D. E. Stone for making mass collections; P. J. Gautrey for searching the Gray-Darwin correspondence at Cambridge University; Rick Kawin for identifying the thrips; Patricia Watters for her many hours of laboratory studies; and to W. L. Culberson for spending many long hours with me in the field and for otherwise assisting this study. The work reported here was supported in part by grants from the National Science Foundation. position based on studies conducted in the northern Piedmont region of North Carolina. MATERIALS AND METHODs-This study was initiated in 1962, when the floral morphology and compatibility relationships of plants in a population of Hedyotis caerulea on the Duke University campus, Durham County, North Carolina, were investigated (Ornduff, 1977). Since then, I have studied a number of other popualtions elsewhere in the northern Piedmont region of North Carolina over a series of several successive years (Table 1). Pollen size was determined by using acetolyzed pollen grains, and pollen viability was determined by scoring the proportion of stained pollen grains mounted in aniline bluelactophenol. Pollen production of the two morphs was determined by acetolyzing undehisced anthers and estimating the total number of grains per flower by use of a hemacytometer. Compatibility relationships, determined by observing seed set following interand intramorph pollinations, have been reported elsewhere (Ornduff, 1977). Representation of the two morphs in populations was determined by a random sampling of individuals, by an attempt to sample all individuals in the popula-