An Interspecific Backcross of Lycopersicon esculentum × L. hirsutum: Linkage Analysis and a QTL Study of Sexual Compatibility Factors and Floral Traits

A BC1, population of the self-compatible tomato Lycopersicon esculentum and its wild self-incompatible relative L. hirsutum f. typicum was used for restriction fragment length polymorphism linkage analysis and quantitative trait loci (QTL) mapping of reproductive behavior and floral traits. The self-incompatibility locus, S, on chromosome 1 harbored the only QTL for self-incompatibility indicating that the transition to self-compatibility in the lineage leading to the cultivated tomato was primarily the result of mutations at the S locus. Moreover, the major QTL controlling unilateral incongruity also mapped to the S locus, supporting the hypothesis that self-incompatibility and unilateral incongruity are not independent mechanisms. The mating behavior of near-isogenic lines carrying the L. hirsutum allele for the S locus on chromosome 1 in an otherwise L. esculentum background support these conclusions. The S locus region of chromosome 1 also harbors most major QTL for several floral traits important to pollination biology (e.g., number and size of flowers), suggesting a gene complex controlling both genetic and morphological mechanisms of reproduction control. Similar associations in other flowering plants suggest that such complex may have been conserved since early periods of plant evolution or else reflect a convergent evolutionary process.

Brassica napus Sources of Resistance to Black Rot in Crucifers and Inheritance of Resistance

Resistance to black rot caused by the bacterium Xanthomonas campestris pv. campestris was studied in Brassica oleracea, B. campestris, and B. napus. Two accessions of B. napus, PI 199947 and PI 199949, exhibited the highest resistance so far found in cultivated Brassica spp. In B. napus, the high level of resistance was conferred by one dominant gene. In B. campestris, two Chinese cabbage accessions showed quantitative inheritance for moderate levels of resistance. Resistance was transferred to B. campestris from B. napus, but a unilateral incongruity was observed for black rot and morphology, but not for stem color or bolting. The bridge line 15 was used to transfer resistance to B. oleracea.

THE UNILATERAL INTERSPECIFIC CROSSING BARRIER IN LYCOPERSICON IS NOT SELF INCOMPATIBILITY

Little is known about the mechanisms controlling interspecific barriers, unlike the well studied intraspecific barrier, self incompatibility (SI),. A unilateral crossing barrier (unilateral incongruity - UI) exists among the Lycopersicon species, in which crossing is impeded or prevented in one direction. Since both UI and SI can give unilateral differences in seed set, suggestions have been made that UI and SI are functionally related. L. pennellii LA716 is self-compatible, unlike the other accessions which are SI, but LA716 still exhibits UI with L. esculentum (esc). We observed the development of pollen tubes in self and cross pollinations of LA716, esc and SI accessions of L. pennellii (pen). Selfed pollen tubes in esc were at the ovary in 24 hours, while pen were 1/2 way down the style and in LA716 the pollen had not germinated. By 48 hours, the pollen tubes in LA716 were in the ovary and growth had halted in pen styles. Crosses with LA716 pollen on esc and pen resulted in pollen tube growth starting within 24 hours continuing to the ovary. Thus, UI is not a SI response and LA716 shows a delayed pollen germination and growth unlike the other Lycopersicon species examined.