Abstract
Meiotic pairing between homoeologous chromosomes in polyploid wheat is inhibited by the Ph1 locus on the long arm of chromosome 5 in the B genome. Aegilops speltoides (genomes SS), the closest relative of the progenitor of the wheat B genome, is polymorphic for genetic suppression of Ph1. Using this polymorphism, two major suppressor loci, Su1-Ph1 and Su2-Ph1, have been mapped in Ae. speltoides. Su1-Ph1 is located in the distal, high-recombination region of the long arm of the Ae. speltoides chromosome 3S. Its location and tight linkage to marker Xpsr1205-3S makes Su1-Ph1 a suitable target for introgression into wheat. Here, Xpsr1205-3S was introgressed into hexaploid bread wheat cv. Chinese Spring (CS) and from there into tetraploid durum wheat cv. Langdon (LDN). Sequential fluorescence in situ hybridization and genomic in situ hybridization showed that an Ae. speltoides segment with Xpsr1205-3S replaced the distal end of the long arm of chromosome 3A. In the CS genetic background, the chromosome induced homoeologous chromosome pairing in interspecific hybrids with Ae. peregrina but not in progenies from crosses involving alien disomic substitution lines. In the LDN genetic background, the chromosome induced homoeologous chromosome pairing in both interspecific hybrids and progenies from crosses involving alien disomic substitution lines. We conclude that the recombined chromosome harbors Su1-Ph1 but its expression requires expression of complementary gene that is present in LDN but absent in CS. We suggest that it is unlikely that Su1-Ph1 and ZIP4-1, a paralog of Ph1 located on wheat chromosomes 3A and 3B and Ae. tauschii chromosome 3D, are equivalent. The utility of Su1-Ph1 for induction of recombination between homoeologous chromosomes in wheat is illustrated.
Highlights
In most allopolyploid plants, only homologous chromosomes pair in meiosis and only bivalents are present at metaphase I (MI); pairing between homoeologous chromosomes is excluded (Jenczewski and Alix, 2004)
Selection based only on meiotic phenotype would require a testcross with Ae. peregrina, or a similar tester, each backcross generation to ascertain that Ph1 is suppressed
Heterozygosity for translocations resulting from recombination between homoeologous chromosomes during backcrossing or from chromosome breakage and non-homologous end-joining due to the activity of gametocidal genes (Tsujimoto and Tsunewaki, 1984; Kota and Dvorak, 1988; Marais et al, 2010) could potentially lead to mistaking such multivalent pairing for homoeologous pairing
Summary
Only homologous chromosomes pair in meiosis and only bivalents are present at metaphase I (MI); pairing between homoeologous chromosomes (heterogenetic chromosome pairing) is excluded (Jenczewski and Alix, 2004). While the chromosomes of these diploid species extensively pair in hybrids among them, virtually no chromosome pairing take places in haploids derived from polyploid wheat (Kimber and Riley, 1963; McGuire and Dvorák, 1982; Jauhar et al, 1991, 1999). This paradox is caused by the expression of the Ph1 locus on chromosome 5B of tetraploid and hexaploid wheat, which prevents pairing between homoeologous chromosomes (Okamoto, 1957; Riley and Chapman, 1958; Sears and Okamoto, 1958). If Ph1 is absent due to aneuploidy (Riley, 1960) or a deletion, such as ph1b in hexaploid wheat (Sears, 1977) and ph1c in tetraploid wheat (Giorgi, 1978), meiotic pairing of homoeologous chromosomes is restored
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