ALLOZYMES OF GALAPAGOS TOMATOES: POLYMORPHISM, GEOGRAPHIC DISTRIBUTION, AND AFFINITIES.

Abstract

As the research of many investigators has recently revealed, studies of variation in allozymes (allelically determined variants of isozymes) provide many advantages for analyzing genetic variation in natural populations. Several conclusions reached in earlier studies concerning aspects of the population biology of the Galapagos tomatoes demanded further, more rigorous testing, for which allozyme analysis is the ideal procedure. For reasons previously presented (Rick, 1971), all Galapagos tomatoes are logically treated as a single species, Lycopersicon cheesmanii Riley. Even though relatively polymorphic, all tested, indigenous populations possess a common set of several morphological and physiological traits but differing from those of the continental species. No barriers to crossing between any tested Galapagos accessions have been encountered, but obstacles to intercrossing and genetic 'disharmonies' in the hybrids would probably interfere with introgression between L. cheesmanii and other species if they were sympatric. Subspecific differentiation in L. cheesmanii has proceeded to the extent that several forms can be distinguished. The most distinct and widespread is f. minor (designated by italics in Fig. 1), which is characterized by short internodes, highly divided leaves, dense pubescence of large trichomes, large accrescent calyx in later stages of fruit development, and additional floral and fruit traits. Although some of these traits are known in other biotypes, they almost always appear singly. In previous studies (Rick, 1963, 1966), evidence of the following kinds was encountered to suggest that L. cheesmanii is highly autogamous and that its natural populations may indeed exist in a virtually pure-line condition. (1) Floral structure is adapted to automatic self-pollination. (2) Experiments in culture and in the native habitat show that automatic self-pollination can take place without the intervention of any kind of pollen vector. (3) Little or no activity of insect pollen vectors was observed under natural conditions. (4) With few exceptions, a high level of uniformity in morphological characters was observed within populations. (5) High uniformity is also characteristic of the progenies grown from seeds harvested from plants in natural populations. (6) Welldistinguished, sometimes seemingly deleterious phenotypes are often fixed in entire populations. Allozymic variation was studied not only for the aforementioned purpose but also to investigate the extent of genetic differentiation between populations and races as well as to compare the zymograms of L. cheesmanii with those of the most closely allied species from continental South America. Historically, various names have been applied to the islands of the Galapagos group (Wiggins and Porter, 1971). Despite a tendency in the scientific literature to use the English names, we prefer the official names of the Republic of Ecuador.