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Abstract:
We have genotyped at high resolution and with common markers over 7000 F2 plants derived from pair-wise crosses between 18 accessions. Most populations showed contained markers with significant deviation from the expected 1: 2: 1 Mendelian ratio, indicating that segregation distortion is widespread. The most striking examples are seen in a population derived from Lov-5 (Sweden) and Shahdara (Tadjikistan); they are not caused by seed lethality, as F 1 siliques are full. Both distorted regions map to chromosome 1: on the upper arm, plants homozygous for the Lov-5 allele are virtually absent, while the lower arm is almost devoid of plants homozygous for Sha (n plants = 443). Reciprocal crosses between Lov-5 x Sha F 1 plants and each parent demonstrate that distortion is only associated with F1 pollen. Distortion on the upper arm of chromosome 1 is only seen when Sha is used as pollen recipient; this locus favors Sha pollen over Lov-5. Distortion on the lower arm is seen irrespective of the parent used as pollen recipient, and represents a case of pollen competition, whereby only pollen carrying a Lov-5 allele at the causal locus gives rise to seed. The promoter of the likely causal locus shows extensive divergence in Lov-5 from the Col-0 and Sha alleles, and appears more similar to the orthologous A. lyrata promoter. A second, pollen-expressed cryptic locus may also be important for the expression of distortion, as a cross between Lov-5 and Bay-0 leads to limited distortion in the F2 generation. The Col-0 and Ler accessions do not carry this cryptic modifier, as a T-DNA loss-of-function allele for the candidate locus in a Col-0 background does not result in distortion when crossed to Ler. Identification of all causal loci is underway, and will yield critical information on the non-random segregation of alleles in F2 and more advanced populations.
