DIFFERENTIATION BETWEEN WHEAT CHROMOSOMES 4B AND 4D

A linkage map based on homoeologous recombination, induced by the abse nce of the Phl locus, between chromosome 4D of Triticum aestivum L. (g enomes AABBDD) and chromosome 4B of T. turgidum L. (genomes AABB) was compared with a linkage map of chromosome 4A(m) of T. monococcum L. an d a consensus map of chromosomes 4B and 4D of T. aestivum based on hom ologous recombination. The 4D/4B homoeologous map was only one-third t he length of the homologous maps and all intervals were reduced relati ve to the 4B-4D consensus map. After the homoeologous map was correcte d for this overall reduction in recombination, the distribution of rec ombination in the short arm was similar in both types of maps. In the long arm, homoeologous recombination declined disproportionally in the distal to proximal direction. This gradient was shown to be largely c aused by severe segregation distortion reflecting selection against 4D genetic material. The segregation distortion had a maximum that coinc ided with the centromere and likely had a polygenic cause. Chromosomes 4D and 4B were colinear and recombination between them occurred in al most all intervals where homologous recombination occurred. These find ings suggest that these chromosomes are not differentiated structurall y and that the differentiation is not segmental. In the presence of Ph l, metaphase I chromosome pairing between chromosomes composed of homo logous and differentiated regions correlated with the lengths of the h omologous regions. No compensatory allocation of crossovers into the h omologous regions was detected. In this respect, the present results a re in dramatic contrast with the crossover allocation into the pseudoa utosomal region in the mammalian male meiosis.