PROTEIN POLYMORPHISM IN THE NARROW ENDEMIC OENOTHERA ORGANENSIS

Oenothera organensis Munz has been an enigma because of its small population size but rich self-incompatibility (S) gene polymorphism. This perennial is known solely from the Organ Mountains, New Mexico, at elevations between 6,000 and 7,500 ft; and occupies an area of about 18 sq. mi. Emerson (1939, 1940) sampled four populations and found over 45 S-alleles whose distribution was rather homogeneous. Forty-five alleles is very high in view of Emerson's notion that the total population of Oe. organensis was less than 500 plants. There have been several attempts to explain how so many alleles each could be maintained at a low frequency in a species, with so few individuals (Wright, 1939, 1960, 1965; Fisher, 1958; Moran, 1962; Crosby, 1966). Is this very narrow endemic (rich in Salleles) rich or depauperate in genetic variation at other loci? Moreover is the spatial organization of variation at other loci similar to that at the S-locus? The purpose of our study was to obtain answers to these questions. As a prelude to stating our expectations and considering our findings, it is important to recognize a number of aspects of the ecology of the species. Recent exploration of the Organ Mountains by Ritter revealed that Oe. organensis is abundant in canyons that have good drainage. All of the populations known to date, and locations where others are likely, are noted in Figure 1. The species may be composed of 5,000 plants. The largest number of plants occur on the east side of the mountains which is rather mesic in many areas. The dispersal of pollen within and among populations is accomplished by strongflying hawkmoths (Hyles lineata, Manduca quinquemaculata, Sphinx chersis). Seeds have no obvious special dispersal adaptations. Local dispersal presumably is by water and small mammals or birds. Interpopulation seed dispersal is probably accomplished by deer, which heavily browse inflorescences and capsules. The sites of Oenothera are the only watering holes for deer in their peregrinations over ridges between canyons. Given the information on S-locus polymorphism, the distribution of populations and the type of pollen and seed vectors, what may we expect to find for the genome as a whole? Clearly S-allele polymorphism is not indicative of overall genetic variation. Polymorphism at the Slocus is protected in part through frequency-dependent selection; the rare alleles tend to be at an advantage (Wright, 1969). This form of selection tends to retard the decay of genetic variability and insure the retention of at least a few alleles. At other loci, the decay of variability is likely to be rapid, being dictated by the size and number of populations and the amount of isolation and differential selection between the populations. If there is little genetic subdivision of the species, as is suggested by the S-alleles, then the species would be a rather poor repository for genetic variation (Wright, 1948, 1951). In view of its narrow endemism, we hypothesize that Oe. organensis is monomorphic at most allozyme loci and has few alleles at polymorphic loci. The spatial distribution of alleles at other loci is likely to be governed by the same factors operative on S-alleles, especially the pollen and seed flow distributions. Since pollen is carried by strong