Genetics of isozyme loci in Brassica campestris L. and in the progeny of a trigenomic hybrid between B. napus L. and B. campestris L.

F2 progeny of Brassica campestris crosses were analyzed for single-locus inheritance of glucosephosphate isomerase, leucine aminopeptidase, 6-phosphogluconate dehydrogenase, phosphoglucomutase, and shikimate dehydrogenase enzymes. In most of the F2 families, the observed inheritance data for six polymorphic isozyme loci coincided well with the ratios expected under Mendelian segregation of either codominant alleles or dominant-recessive alleles when a null allele was involved. Complete linkage was observed for one locus pair (Lap-2A/6Pgd-2Ac), with the recombination frequency estimated to be r ≈ 0.000. From isozyme analyses made on resynthesized Brassica napus (AACC) and the actual parents B. campestris (AA) and B. alboglabra (CC) and on a trigenomic hybrid (AAC) between B. napus and B. campestris, it was possible to recognize A and C genome specific isozyme loci through the nonoverlapping electrophoretic mobilities of alleles characteristic of each genome. The trigenomic hybrid was selfed and genetic analyses of the offspring indicated that the A genome specific isozyme loci displayed a normal disomic inheritance. The C genome specific isozyme loci, on the other hand, showed nonrandom loss in the aneuploid offspring, thereby indicating the nonrandom loss of C genome chromosomes. At least 4 of the 8 C genome specific isozyme loci studied were located on separate chromosomes. The apparent occurrence of multiplicates of certain isozyme loci supports the concept that duplication of structural nuclear genes prevails in the diploid Brassica genomes.Key words: isozymes, Brassica, inheritance, trigenomic hybrid (2n = 29, AAC), genome-specific isozyme loci.

Large disparity in recombination fractions reported in Picea mariana for the Aat-1/Pgi-2 linkage group

A survey of 84 trees in a natural population of Picea mariana located near St. John's, Newfoundland, yielded 3 trees heterozygous for the Aat-1/Pgi-2 linked locus pair. Segregation analysis of this linkage group in 330 megagametophytes resulted in an estimated pooled recombination fraction of 0.015. This is the lowest known recombination fraction for this linkage group in the Pinaceae. It is in contrast with earlier reports of recombination fractions of 0.155 and 0.250 from other populations of P. mariana in eastern Canada. The presence of substantial variation in recombination fractions in populations from eastern Canada precludes a regional selection–adaptation explanation. The cause of this variation is unknown. Substantial variation, both temporal and spatial, in other linkage groups suggests that the basis of linkage variation is complex and in need of further investigation in the Pinaceae.

Inheritance and linkage relationships of allozymes of eastern white cedar (Thuja occidentalis) in northwestern Ontario

Allozyme variants of 12 enzyme systems were examined in seed tissues of eastern white cedar (Thuja occidentalis L.), using starch gel electrophoresis. Nine loci were polymorphic and deviation from a 1:1 segregation ratio was observed between two of three alleles at one locus (Mdh-1). Of the possible 36 locus-pair combinations, 23 could be tested for linkage. Significant linkage was detected for three pairs of loci (Aat-1/Mdh-1, Aat-1/Pgm, and Idh-2/Me). Trees jointly segregating for Aat-1 and Pgm fell into two classes, one with a low recombination frequency (9.1%) and the other with a higher recombination frequency (26.6%). An inversion polymorphism is a possible cause of this linkage heterogeneity observed among trees.Key words: allozymes, isozymes, inheritance, linkage, eastern white cedar, northern white cedar, Thuja occidentalis L., Cupressaceae.

Linkage disequilibrium in natural and experimental populations of Drosophila melanogaster.

We have studied linkage disequilibrium in Drosophila melanogaster in two samples from a wild population and in four large laboratory populations derived from the wild samples. We have assayed four polymorphic enzyme loci, fairly closely linked in the third chromosome: Sod Est-6, Pgm, and Odh. The assay method used allows us to identify the allele associations separately in each of the two homologous chromosomes from each male sampled. We have detected significant linkage disequilibrium between two loci in 16.7% of the cases in the wild samples and in 27.8% of the cases in the experimental populations, considerably more than would be expected by chance alone. We have also found three-locus disequilibria in more instances than would be expected by chance. Some disequilibria present in the wild samples disappear in the experimental populations derived from them, but new ones appear over the generations. The effective population sizes required to generate the observed disequilibria by randomness range from 40 to more than 60,000 individuals in the natural population, depending on which locus pair is considered, and from 100 to more than 60,000 in the experimental populations. These population sizes are unrealistic; the fact that different locus-pairs yield disparate estimates within the same population argues against the likelihood that the disequilibria may have arisen as a consequence of population bottlenecks. Migration, or population mixing, cannot be excluded as the process generating the disequilibria in the wild samples, but can in the experimental populations. We conclude that linkage disequilibrium in these populations is most likely due to natural selection acting on the allozymes, or on loci very tightly linked to them.