Genetic and physical mapping of homoeologous recombination points involving wheat chromosome 2B and rye chromosome 2R.

Wide hybrids have been used in generating genetic maps of many plant species. In this study, genetic and physical mapping was performed on ph1b-induced recombinants of rye chromosome 2R in wheat (Triticum aestivum L.). All recombinants were single breakpoint translocations. Recombination 2RS-2BS was absent from the terminal and the pericentric regions and was distributed randomly along an intercalary segment covering approximately 65% of the arm's length. Such a distribution probably resulted from structural differences at the telomeres of 2RS and wheat 2BS arm that disrupted telomeric initiation of pairing. Recombination 2RL-2BL was confined to the terminal 25% of the arm's length. A genetic map of homoeologous recombination 2R-2B was generated using relative recombination frequencies and aligned with maps of chromosomes 2B and 2R based on homologous recombination. The alignment of the short arms showed a shift of homoeologous recombination toward the centromere. On the long arms, the distribution of homoeologous recombination was the same as that of homologous recombination in the distal halves of the maps, but the absence of multiple crossovers in homoeologous recombination eliminated the proximal half of the map. The results confirm that homoeologous recombination in wheat is based on single exchanges per arm, indicate that the distribution of these single homoeologous exchanges is similar to the distribution of the first (distal) crossovers in homologues, and suggest that successive crossovers in an arm generate specific portions of genetic maps. A difference in the distribution of recombination between the short and long arms indicates that the distal crossover localization in wheat is not dictated by a restricted distribution of DNA sequences capable of recombination but by the pattern of pairing initiation, and that can be affected by structural differences. Restriction of homoeologous recombination to single crossovers in the distal part of the genetic map complicates chromosome engineering efforts targeting genes in the proximal map regions.

Genetics of adaptive radiation in Hawaiian species of Tetramolopium (Asteraceae). III. Evolutionary genetics of sex expression.

Despite numerous studies of speciation on oceanic islands, few insights exist on the genetic changes involved in the origin and diversification of island taxa. Here we report a genetic analysis of the evolutionary change in sex expression in Hawaiian Tetramolopium. The most diverse clade in the genus is characterized by a monoecious breeding system. The breeding system resulted from a change in sex expression in disc florets from the ancestral hermaphroditic condition to the derived male state. Analysis of an F2 population from a cross between the two forms of sex expression indicates two regions of the Tetramolopium linkage map are associated with the loss of female function in disc florets. Quantitative trait locus mapping of the two linkage groups confirms that two loci control 56% of the phenotypic variation of the trait in the F2 population. Additive and dominance effects are apparent but no statistical evidence of epistasis was found. Several related reproductive traits also have few genetic associations on the linkage map, but are generally distinct from the control of sex expression. Although modifier loci are likely to be involved, the apparent simple genetic change underlying sex expression parallels a major evolutionary diversification in Hawaiian Tetramolopium and may have initiated the divergence of this novel clade.

Genetics of adaptive radiation in Hawaiian and Cook Islands species of Tetramolopium (Asteraceae). II. Genetic linkage map and its implications for interspecific breeding barriers.

In a study of the genetic mechanisms associated with adaptive radiation in Hawaiian Tetramolopium, a genetic linkage map was constructed in an interspecific cross. A total of 125 RFLP and RAPD markers were mapped into 117 different loci on nine linkage groups for a map length of 665.7 cM. Segregation distortion occurred in 49% of the mapped probes, located primarily in four linkage groups. High percentages of one parental species genotype (Tetramolopium rockii) were recovered in three of these blocks and the second parental species (T. humile) in the remaining block. The high degree of distorted segregation suggests the buildup of internal crossing barriers, even though island plant species are typically characterized as highly cross compatible with few to no internal crossing barriers. This work and a review of previous crossing studies in island plants show that internal (postmating) crossing barriers do exist. The weak crossing barriers have likely been overlooked because the main focus has been on diversification and speciation through adaptation to extremely diverse environments.

Genetics of adaptive radiation in Hawaiian and Cook Islands species of Tetramolopium (Asteraceae; Astereae). I. Nuclear RFLP marker diversity.

Thirty-three nuclear RFLP (restriction fragment length polymorphism) probes were used to study genetic diversity in Hawaiian and Cook Islands species of Tetramolopium for comparison with previous morphological and isozyme studies and to provide greater resolution of the events associated with adaptive radiation in the genus. Levels of RFLP diversity are greater than those reported for isozymes, yet are still low in comparison to continental species. Genetic differentiation is greatest among species in sections rather than among sections and is concordant with the hypothesis of phyletic sorting of initial variability as suggested for morphological traits. Hypothesized introgression between T. lepidotum and T. filiforme is supported, but the evidence suggests bidirectional gene flow. Systematic relationships derived from the data agree with hypotheses based on morphology in the placement of populations within their respective species and the recognition of three main lineages within Hawaii. Inclusion of the Cook Islands species, however, renders section Tetramolopium paraphyletic, contradicting morphological, ecological, and crossing evidence. Interpreting these results in light of evidence from previous studies, the genetic diversity and relationships seen among species and sections of Hawaiian and Cook Islands Tetramolopium reflect the recent and rapid evolution of this group, limited addition of new variability, and phyletic sorting.

Distribution of spontaneous plant hybrids.

Natural hybridization is a relatively common feature of vascular plant species and has been demonstrated to have played an important role in their evolution. Nonetheless, it is not clear whether spontaneous hybridization occurs as a general feature of all plant families and genera or whether certain groups are especially prone to spontaneous hybridization. Therefore, we inspected five modern biosystematic floras to survey the frequency and taxonomic distribution of spontaneous hybrids. We found spontaneous hybridization to be nonrandomly distributed among taxa, concentrated in certain families and certain genera, often at a frequency out of proportion to the size of the family or genus. Most of these groups were primarily outcrossing perennials with reproductive modes that stabilized hybridity such as agamospermy, vegetative spread, or permanent odd polyploidy. These data suggest that certain phylogenetic groups are biologically predisposed for the formation and maintenance of hybrids.

Construction of an RFLP map in sorghum and comparative mapping in maize.

An F2 population derived from a cross between Sorghum bicolor ssp. bicolor ('CK60') and Sorghum bicolor ssp. drummondii ('PI229828') was used to develop an RFLP genetic linkage map of sorghum. The map consists of 201 loci distributed among 10 linkage groups covering a map distance of 1530 cM, with an average 8 cM between adjacent loci. Maize genomic probes (52), maize cDNA probes (124), and sorghum genomic probes (10) were used to define the loci (55, 136, and 10, respectively). Ninety-five percent of the loci fit expected segregation ratios. The loci with distorted segregation ratios were confined almost exclusively to a region of one linkage group. Comparison of sorghum and maize maps indicated high correspondence between the two genomes in terms of loci order and genetic distance. Many loci linked in maize (45 of 55) were also linked in sorghum. Instances of both conserved and rearranged locus orders were detected.

Comparative genome mapping of Sorghum and maize.

Linkage relationships were determined among 85 maize low copy number nuclear DNA probes and seven isozyme loci in an F2 population derived from a cross of Sorghum bicolor ssp. bicolor x S. bicolor ssp. arundinaceum. Thirteen linkage groups were defined, three more than the 10 chromosomes of sorghum. Use of maize DNA probes to produce the sorghum linkage map allowed us to make several inferences concerning processes involved in the evolutionary divergence of the maize and sorghum genomes. The results show that many linkage groups are conserved between these two genomes and that the amount of recombination in these conserved linkage groups is roughly equivalent in maize and sorghum. Estimates of the proportions of duplicated loci suggest that a larger proportion of the loci are duplicated in the maize genome than in the sorghum genome. This result concurs with a prior estimate that the nuclear DNA content of maize is three to four times greater than that of sorghum. The pattern of conserved linkages between maize and sorghum is such that most sorghum linkage groups are composed of loci that map to two maize chromosomes. This pattern is consistent with the hypothesized ancient polyploid origin of maize and sorghum. There are nine cases in which locus order within shared linkage groups is inverted in sorghum relative to maize. These may have arisen from either inversions or intrachromosomal translocations. We found no evidence for large interchromosomal translocations. Overall, the data suggest that the primary processes involved in divergence of the maize and sorghum genomes were duplications (either by polyploidy or segmental duplication) and inversions or intrachromosomal translocations.