Abstract
BackgroundMultiple hybridization events gave rise to pentaploid dogroses which can reproduce sexually despite their uneven ploidy level by the unique canina meiosis. Two homologous chromosome sets are involved in bivalent formation and are transmitted by the haploid pollen grains and the tetraploid egg cells. In addition the egg cells contain three sets of univalent chromosomes which are excluded from recombination. In this study we investigated whether differential behavior of chromosomes as bivalents or univalents is reflected by sequence divergence or transcription intensity between homeologous alleles of two single copy genes (LEAFY, cGAPDH) and one ribosomal DNA locus (nrITS).ResultsWe detected a maximum number of four different alleles of all investigated loci in pentaploid dogroses and identified the respective allele with two copies, which is presumably located on bivalent forming chromosomes. For the alleles of the ribosomal DNA locus and cGAPDH only slight, if any, differential transcription was determined, whereas the LEAFY alleles with one copy were found to be significantly stronger expressed than the LEAFY allele with two copies. Moreover, we found for the three marker genes that all alleles have been under similar regimes of purifying selection.ConclusionsAnalyses of both molecular sequence evolution and expression patterns did not support the hypothesis that unique alleles probably located on non-recombining chromosomes are less functional than duplicate alleles presumably located on recombining chromosomes.
Highlights
Multiple hybridization events gave rise to pentaploid dogroses which can reproduce sexually despite their uneven ploidy level by the unique canina meiosis
One to three fragments were detected in the digestions of genomic DNA by six different enzymes hybridized against probes of LEAFY or cytosolic glyceraldehyde 3-phosphate dehydrogenase (cGAPDH)
The maximum number of three fragments within the digestions did not contradict against the expectation for LEAFY and cGAPDH to have one copy per each dogrose genome, because we expected a maximum number of five bands in pentaploids
Summary
Multiple hybridization events gave rise to pentaploid dogroses which can reproduce sexually despite their uneven ploidy level by the unique canina meiosis. Section Caninae originated by multiple hybridization events [9] and overcame the sterility bottleneck due to odd ploidy by the development of a unique meiosis mechanism regaining sexual reproduction [10,11,12,13]. This meiotic system is unique in plants, but other meiosis systems leading to comparable effects have been observed e.g. in the sexual triploid plant Leucopogon juniperinus R.Br. High ploidy levels and sexuality have probably been the prerequisites for the evolutionary success of dogroses after the retreat of Pleistocenic ice shields, because dogroses are very widely spread in Central Europe and occur on a broad range of different habitats, whereas diploid and tetraploid species of other sections of Rosa are mainly found in glacial refugia [15]
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