THE TOMATO Ge LOCUS: LINKAGE RELATIONS AND GEOGRAPHIC DISTRIBUTION OF ALLELES

Abstract

RASTICALLY distorted segregation of certain marker genes on chromosome D4 of the tomato (Lycopersicon esculentzrm Mill.) prompted an earlier investigation (RICK 1966) of a series of alleles that effect gamete elimination. Three alleles-Ge", GeC, and GeP-are known at this locus, but gametes are eliminated only in Gec/Gep heterozygotes; both male and female Gee gametes are aborted; the interaction is approximately 95% penetrant. Segregation of linked genes is modified in such heterozygotes, the degree of distrxbance depending on the tight-ness of the linkage. Preliminary data placed the locus of Ge in the vicinity of fuZ and w-4, possibly within the proximal heterochromatin of chromosome 4. Studies of the Ge locus were continued in order to (1) position its locus more precisely and (2) determine which alleles are present in samples o€ wild and cultivated tomatoes from various geographic areas. The first objective was pursued with hope of learning more precisely what relationship, if any, the locus of Ge might have with heterochromatin and the second to ascertain what light might be shed on the relationships amongst tomato cultivars and between them and related wild species. MATERIALS-4ND METHODS LinkagP: Since Ge alleles do not condition any known phenotypic effects, segregation at this locus cannot be scored directly; it is necessary instead to resort to observation of closely linked marker genes. Segregation of the latter can be exploited in two ways: (1) to measure linkage as a function of the degree of distortion (as in RICK 1966) or (2) to determine the allelic constitution of individuals by means of progeny tests. The latter method was adopted for the new studies because it gives more precise estimates, albeit at the cost of testing many progenies in an additional generation. The precision of such tests is improved by the simultaneous segregation of several marker genes. Accordingly, a cross was made between our original stock of Gep-w-4 and ful-GeC-ra-e, a new combination synthesized for this purpose. The most recent summary of linkage and cytological interrelationships is presented in Figure 1. The recessive marker genes were selected for their well-defined phenotypes expressed in early seedling development. e (entire) : leaves less divided and less serrate than normal. ful (fulgens) : foliage uniformly bright yellow-green. ra (rava) : leaves convexly recurved, elongate trichomes. w-4 (wiry-4) : leaf margins eroded, becoming progressively more extreme with growth. afl (albifolium) : cotyledons and first true leaves white, turning green. The F, of this cross had normal phenotype, nearly 50% gamete sterility, and, after selfing, produced the large quantity of seed required for the F,.