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An electrophoretic analysis of histone Hi of Pisum sativum L. was carried out using the collection of 883 accessions of cultivated peas originating from different regions of the Old World. The closely linked genes of five Hi subtypes (2-6) form a cluster, the gene of subtype 1 being located on another chromosome. The fast allelic variant of subtype 1 was not observed to the north of the 44th parallel. The frequency of allele 1 of subtype 5 displays a latitudinal dine and is strongly correlated with the sum of aerial temperatures over the vegetational period. Allele 1 of subtype 6 prevails all over the Old World except Central Asia and China, where allele 3 predominates. From this area allele 3 frequency forms a declining gradient. Alleles of subtypes 2, 3 and 4 exhibit no regularities in their geographic patterns. The data indicate that alleles of Hi subtype 5 and, possibly, subtype 1 in garden pea were subjected to climatically-dependent natural selection under conditions of primitive farming (without conscious selection).

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The pea genome contains seven histone H1 genes encoding different subtypes. Previously, the DNA sequence of only one gene, His1, coding for the subtype H1-1, had been identified. We isolated a histone H1 allele from a pea genomic DNA library. Data from the electrophoretic mobility of the pea H1 subtypes and their N-bromosuccinimide cleavage products indicated that the newly isolated gene corresponded to the H1-5 subtype encoded by His5. We confirmed this result by sequencing the gene from three pea lines with H1-5 allelic variants of altered electrophoretic mobility. The allele of the slow H1-5 variant differed from the standard allele by a nucleotide substitution that caused the replacement of the positively charged lysine with asparagine in the DNA-interacting domain of the histone molecule. A temperature-related occurrence had previously been demonstrated for this H1-5 variant in a study on a worldwide collection of pea germplasm. The variant tended to occur at higher frequencies in geographic regions with a cold climate. The fast allelic variant of H1-5 displayed a deletion resulting in the loss of a duplicated pentapeptide in the C-terminal domain. Heredity (2005) 94, 582-588.

It is hypothesized that, in plants, genetically empty B chromosomes may originate from the extra chromosome (E) of tertiary trisomics if (i) the region of basic chromosomes homologous to the E (H-region) harbors a sporophytic lethal covered by the wild-type allele in E, and (ii) crossing-over between E and the H-region is suppressed. Under these conditions, most loss-of-function mutations occurring in the H-region are deleterious for haploid gametophytes, whereas those occurring in E are neutral or advantageous for hyperploid (n þ 1) gametophytes. As a result, natural selection at the gametophyte level can lead to the degeneration of E, leaving the H-region intact. Using Hammarlund translocation T(3-6)a, we synthesized two trisomic lines of the garden pea (Pisum sativum L.), where E was composed of the short arms of chromosomes 3 and 6 and the H-region carried recessive markers. In the trisomic line TRIS, we found few crossovers between E and the H-region. In the trisomic line TRUST, obtained after a change of basic chromosome constitution, recombination in this region was completely suppressed. After induction in the H-region of TRUST of a recessive sporophytic mutation rmv, two 15-chromosome lines of stable trisomics were established. One of them passed 11 generations, having produced more than 6000 individuals, all of them trisomic, and E remained present as a single element with no pairing partners. No tetrasomics were dete-cted in these lines. If such trisomics occurred in nature, their extra chromosomes are likely to become a B chromosome.