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.

Positive performance is reported for centric translocations of chromosome 1 of rye (Secale cereale L.) in bread wheat (Triticum aestivum L.). Objectives were to determine the effects of short arm translocations of rye chromosome 1 (1RS) derived from ‘Kavkaz’ winter wheat, in ‘Pavon’ spring wheat background, on root biomass, water‐use efficiency, and agronomic performance. Pavon and its translocations were evaluated in glasshouse pot experiments in 1997 and 1998 and in field experiments in 1999 and 2000 under well‐watered and droughted treatments. The 1RS translocations in Pavon delayed maturity, reduced plant height in some cases, and increased root biomass. Association between root biomass and grain yield was significant under droughted and under well‐watered conditions. The 1RS translocations increased grain yield and grain weight, especially under well‐watered field conditions. The overall mean grain yield was 4.066 Mg ha−1 for Pavon, 4.895 Mg ha−1 for 1RS.1AL, 4.503 Mg ha−1 for 1RS.1BL, and 4.632 Mg ha−1 for 1RS.1DL. The 1RS translocations, in general, were more tolerant to field environmental stresses than Pavon. These results encourage the development and use of the 1RS.1AL and 1RS.1DL translocations in wheat breeding programs.

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.

Chromosome rearrangements may affect the rate and patterns of gene flow within species, through reduced fitness of structural heterozygotes or by reducing recombination rates in rearranged areas of the genome. While the effects of chromosome rearrangements on gene flow have been studied in a wide range of organisms with monocentric chromosomes, the effects of rearrangements in holocentric chromosomes--chromosomes in which centromeric activity is distributed along the length of the chromosome--have not. We collected chromosome number and molecular genetic data in Carex scoparia, an eastern North American plant species with holocentric chromosomes and highly variable karyotype (2n = 56-70). There are no deep genetic breaks within C. scoparia that would suggest cryptic species differentiation. However, genetic distance between individuals is positively correlated with chromosome number difference and geographic distance. A positive correlation is also found between chromosome number and genetic distance in the western North American C. pachystachya (2n = 74-81). These findings suggest that geographic distance and the number of karyotype rearrangements separating populations affect the rate of gene flow between those populations. This is the first study to quantify the effects of holocentric chromosome rearrangements on the partitioning of intraspecific genetic variance.

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