SPATIAL SEGREGATION OF PINS AND THRUMS IN POPULATIONS OF HEDYOTIS NIGRICANS.

Abstract

The central position of genetic polymorphism in the evolutionary process has elicted extensive experimentation and theoretical exercises on those factors and interactions which temporarily or permanently maintain this state (e.g., Fisher, 1930; Allard et al., 1968; Levins, 1968; Wright, 1969; Dobzhansky, 1970; Ford, 1971). Another aspect of genetic polymorphism, namely, the spatial patterning of (alternate) morphs within local populations attracted little attention until the recent expansion of interest in local genetic differentiation and population subdivision. Population subdivision has been demonstrated through the analysis of morph frequency within a network of quadrats (Epling and Dobzhansky, 1942; Selander, 1970) or political districts (Workman and Niswander, 1970). Fine scale spatial relationships among alternate morphs may be analyzed by the same method used to determine the degree to which two species are intermingled without regard to the pattern of either in relation to the ground. This involves nearest-neighbor relationships of the population members and the concept of segregation (Pielou, 1961). If the number of individuals with nearest-neighbors of the same morph as themselves exceeds expectations based upon an assumption of random distribution, the morphs are referred to as being segregated or clumped, or having contagious distributions. If none of the individuals of one morph has nearestneighbors of the alternative type, segregation is total, otherwise it is referred to as partial. The pin-thrum system in plants is a polymorphism free from the effects of inbreeding and genetic drift. The possibility that pins and thrums might be partially segregated occurred to me several years ago during a nearest-neighbor analysis on heterostylic Lithospermum species in northern Indiana. Unfortunately, the study areas were bulldozed and the matter rested until the Spring of 1973 when I was introduced to a common Texas weed, Hedyotis nigricans. This species is heterostylic and the matter of morph segregation was reopened. Fortuitously, the first few populations examined promised some interesting results, and the study described below commenced. No other heterostylic species were examined and rejected for lack of morph segregation. The objective of this investigation was to determine the spatial relationships of pins and thrum in Hedyotis nigricans (Lam.) Fosb. (Rubiaceae). The investigation was designed with the following questions in mind: Are pins and thrums partially segregated, and what is the degree of that segregation? To what extent does the pattern vary among populations? Does the level of segregation decline when the second and third nearest-neighbors are considered? Is the level of segregation for the first, second and third nearest-neighbor classes density-dependent? Does the pin: thrum ratio depart significantly from the 1: 1 expectation? The species is a common roadside weed in the Southern and Central United States and frequently occurs in dense stands. It is a small, ascending perennial arising from a taproot, and has no mechanism for clone formation by vegetative apomixis. Reproduction is exclusively from seed. Pins and thrums are characterized by differences in