INTERSPECIFIC HYBRIDIZATION, HETEROZYGOSITY AND GENE EXCHANGE IN PHLOX.

The roles of interspecific hybridization and gene exchange in the evolution of plant populations and species have been the subject of intense interest for the past two decades. Whereas great strides have been made in recognizing natural hybridization, little progress has been made in assessing the extent of gene exchange or admixture between species within single populations and across the areas of species contact. The prime factor contributing to this inertia has been our inability to detect and to quantify gene exchange. Many morphological, physiological and chemical expressions of species are controlled by several unlinked genes (Clausen and Hiesey, 1958; Alston, 1967; Wallace et al., 1972). To transfer a multigenic diagnostic feature of one eucaryotic species into the genome of another, all the genes involved must cross the "hybrid bridge" between the species (independently) and then be reunited in one species by recurrent backcrosses. The more unlinked genes that are involved, the less is the likelihood of all genes being transferred and gene exchange being perceived. Since many diagnostic characters are controlled by two or more genes, much gene flow probably goes undetected. The detection and quantification of alien genes is greatly facilitated by the use of electrophoretic techniques. Most enzyme variants or allozymes discerned by this technique are assumed to be under single structural-gene control, inherited in a simple Mendelian fashion, and are identifiable in homozygous or heterozygous states (Shaw, 1965; Shannon, 1968; Scandalios, 1969). Thus enzyme electrophoresis offers great opportunities for recognizing and quantifying the introduction of qualitative alien alleles following hybridization. The primary question being addressed in this paper is: To what extent has gene exchange occurred following hybridization between Phlox drummondii Hook. and P. cuspidata Scheele? Hybridization between these species has been described by Whitehouse (1945) and Erbe and Turner (1962). From a study of morphological characters, they concluded that hybridization between P. drummondii and P. cuspidata leads to extensive introgression. However, a chromatographic and cytological study of hybrid swarms involving these species showed that individuals appear to be sterile F1 hybrids or parental types, and that introgression was absent or occurring at very low levels (Levin, 1967). Phlox drummondii and P. cuspidata are annuals indigenous to central and eastern Texas. They are conspicuous elements of the spring flora and form large discontinuous colonies of various sizes in mesic grasslands, open oak woodlands, fields and roadsides. Although the species abound in similar habitats, they rarely form mixed populations because of well-defined differences in moisture requirements, P. drummondii being the more xeric of the two. Occasionally local disturbance permits the formation of confluent or contiguous populations. Both species flower from March to April, and their fragrant blossoms attract an array of Lepidoptera (Erbe and Turner, 1962; Grant and Grant, 1965). Phlox drummondii is self-incompatible whereas P. cuspidata is self-compatible (Erbe and Turner, 1962; Levin, unpubl.). The latter probably is highly self-fertilizing, because it is self-pollinating and self pollen grains germinate as readily and send tubes down the style as rapidly as outcross pollen (Levin, unpubl.). The barriers to inter-