Combining rare alleles and grouped pollen donors to assign paternity in pollen dispersal studies.

PREMISE: Pollen dispersal plays a critical role in gene flow of seed plants. Most often, pollen dispersal is measured using paternity assignment. However, this approach can be time-consuming because it typically entails genotyping all pollen donors, receptors, and offspring at several molecular markers. METHODS: We developed a faster, simpler protocol to track paternity, using pollen receptors and grouped pollen donors that possess rare alleles. We tested this approach using wind-pollinated Amaranthus tuberculatus and insect-pollinated Solanum lycopersicum. After screening potential markers for rare alleles, we grew both species in experimental arrays under field conditions. RESULTS: All tested A. tuberculatus seeds and 97% of S. lycopersicum fruits could be assigned to the grouped pollen donors using each of two markers. From these results, we could infer paternity of untested offspring and assess pollen dispersal patterns in each array. DISCUSSION: By combining rare alleles and grouped pollen donors, we could assess pollen dispersal for both species and across all arrays after genotyping a small number of pollen donors and a representative subset of offspring. While directly applicable to A. tuberculatus and S. lycopersicum, this approach could be used in other species to assess pollen dispersal under field conditions.

High-density SNP genotyping of tomato (Solanum lycopersicum L.) reveals patterns of genetic variation due to breeding.

The effects of selection on genome variation were investigated and visualized in tomato using a high-density single nucleotide polymorphism (SNP) array. 7,720 SNPs were genotyped on a collection of 426 tomato accessions (410 inbreds and 16 hybrids) and over 97% of the markers were polymorphic in the entire collection. Principal component analysis (PCA) and pairwise estimates of F(st) supported that the inbred accessions represented seven sub-populations including processing, large-fruited fresh market, large-fruited vintage, cultivated cherry, landrace, wild cherry, and S. pimpinellifolium. Further divisions were found within both the contemporary processing and fresh market sub-populations. These sub-populations showed higher levels of genetic diversity relative to the vintage sub-population. The array provided a large number of polymorphic SNP markers across each sub-population, ranging from 3,159 in the vintage accessions to 6,234 in the cultivated cherry accessions. Visualization of minor allele frequency revealed regions of the genome that distinguished three representative sub-populations of cultivated tomato (processing, fresh market, and vintage), particularly on chromosomes 2, 4, 5, 6, and 11. The PCA loadings and F(st) outlier analysis between these three sub-populations identified a large number of candidate loci under positive selection on chromosomes 4, 5, and 11. The extent of linkage disequilibrium (LD) was examined within each chromosome for these sub-populations. LD decay varied between chromosomes and sub-populations, with large differences reflective of breeding history. For example, on chromosome 11, decay occurred over 0.8 cM for processing accessions and over 19.7 cM for fresh market accessions. The observed SNP variation and LD decay suggest that different patterns of genetic variation in cultivated tomato are due to introgression from wild species and selection for market specialization.