A simple method for estimating recombination percentages and linkage intensities from F2 data: examples from Triticum monococcum and other self-fertilizing diploid plant species

Mapping and characterization of the recessive leaf rust resistance gene Lr83 on wheat chromosome arm 1DS

KEY MESSAGE

The leaf rust resistance gene in Thatcher wheat derivative 78-1 was mapped to chromosome 1DS with SNP markers and designated as Lr83. 'Thatcher' wheat near isogenic line RL6149, a putative derivative of Triticum dicoccoides, was previously determined to carry leaf rust resistance gene Lr64 on chromosome arm 6AL and a second gene temporarily named LrX on chromosome arm 1DS. The objective of this study was to map and characterize LrX in a population of recombinant inbred lines (RILs) that segregated for a single gene. Thatcher line 78-1 with LrX was crossed with Thatcher and individual F₂ seedlings and F₆ RILs were evaluated for leaf rust response. The 208 F₂ plants segregated for a single recessive gene and 148 F₆ lines for a single gene. The RILs and parents were characterized by genotyping by sequencing (GBS). Six GBS markers and five Kompetitive Allele-Specific PCR (KASP) markers were used to map LrX on the distal region of chromosome arm 1DS. LrX was 1 centiMorgan (cM) proximal to marker K-IWB38437 and 0.4 cM distal to GBS marker 1D_9037138. Line 78-1 was crossed with Thatcher wheat lines with Lr21, Lr42, and Lr60 for allelism tests. LrX mapped 19.49 cM from Lr21 and 11.93 cM from Lr42. In the cross of line 78-1 with the Thatcher line with Lr60, one recombinant in 1,003 F₂ plants was found. LrX and Lr60 are at tightly linked loci on the distal region of chromosome arm 1DS. The gene in line 78-1 was designated as Lr83. Cytological examination of RL6149 provided no evidence of transfer of a chromosome segment of an A- or B-genome chromosome to chromosome 1D.

Recombination rates of soybean varieties from different periods of introduction and release

Theory predicts that selection for adaptability during the short term also favors selection for a reduced recombination rate in the population. The objective of this study was to test whether the cyclic short-term selection which has taken place in soybean breeding programs in the USA since the introduction of the crop has measurably reduced recombination frequencies. Thirteen soybean varieties separated into four different release periods (prior to 1940, 1940-1954, 1955-1969, after 1970) were evaluated for their recombination frequencies within three locus pairs. Recombination frequencies among the individual varieties ranged from 7.6 to 24.1 % at thep 1 r locus pair, from 20.9 to 30.1 % at thelnp 2 locus pair, and from 28.7 to 41.6% at thedt 1 l 1 locus pair. Recombination frequencies were significantly different among varieties within a release period for thep 1 r andlnp 2 locus pairs, but recombination frequencies did not differ among release periods for any locus pair. Thus, apparently, plant breeders have developed soybean varieties with improved adaptation without influencing recombination rates.