Transposable elements in inbreeding and outbreeding populations.

Y OUNG et al. (1994) recently found that a putative transposable element (Lytl) is present at high abundance in the genomes of individuals sampled from a group of red-fruited species of Lycopersicon, and at a much lower abundance in green-fruited species of this genus. Similar or somewhat lower abundances were found in Solanum species believed to be closely related to Lycopersicon, and even lower abundances were found in less closely related Solanum species. These authors hypothesize that the red-fruited group of Ly-copersicon species evolved from green-fruited species. A s discussed in their paper, this is consistent with other information on these species, including crossability and allozyme genetic distances, though not with data on mitochondrial DNA. The high transposable element abundances in red-fruited species may thus be an ancestral character shared between the species. We here point out that the two groups of species differ in their levels of inbreeding (RICK 1984). The red-fruited species (Lycopersicon escuhtum, cheesmanii, pimpinellifolium) lack self-incompatibility, and several are known to have high autogamous selfing rates, while the green-fruited species include self-incompatible (L. hirsutum, penelli, peruuianum) or facultatively OutcrosS ing (L. chmielewski, chilense) species, as well as the autog-amous L. parvaflmm. Although the outcrossing rates of these species have in general not been estimated using segregation of polymorphic genetic markers in families (see RITLAND and JAIN 1981; RITLAND 1990), inferences from flower sizes and degrees of stigma exsertion are supported by data showing lower genetic diversity within putatively inbreeding populations (RICK 1984; MILLER and TANKSLEY 1990) and, in some cases, by fixation indices consistent with high inbreeding (RICK 1984). According to RICK'S (1984) review, high levels of genetic diversity, suggesting frequent outcrossing, are found in all the green-fruited species included in the study of YOUNG et al. (1994), with the exception of the self-compatible, inbreeding L. hirsutum accession LA407. It is therefore possible that the different transposable element abundances are related to the differences in selfing rates in the two groups of plants. Ectopic exchange between transposable elements (LANGLEY et al. 1988; MONTGOMERY et al. 1991) is a mechanism that might cause such a relationship. Chromosome rearrangements produced by such exchanges during mei-osis probably contribute an important portion of the deleterious effects of transposable elements, and there is evidence that it may be a major mechanism for stabi-WORTH et al. 1994). There is evidence in Drosophila that ectopic exchange is less frequent in chromosomal homozygotes than …