Why the autosomal recombination rate differs between female and male meiosis in most species has been a genetic enigma since the early study of meiosis. Some hypotheses have been put forward to explain this widespread phenomenon and, up to now, only one fact has emerged clearly: In species in which meiosis is achiasmate in one sex, it is the heterogametic one. This pattern, known as the Haldane-Huxley rule, is thought to be a side effect, on autosomes, of the suppression of recombination between the sex chromosomes. However, this rule does not hold for heterochiasmate species (i.e., species in which recombination is present in both sexes but varies quantitatively between sexes) and does not apply to species lacking sex chromosomes, such as hermaphroditic plants. In this paper, we show that in plants, heterochiasmy is due to a male-female difference in gametic selection and is not influenced by the presence of heteromorphic sex chromosomes. This finding provides strong empirical support in favour of a population genetic explanation for the evolution of heterochiasmy and, more broadly, for the evolution of sex and recombination.

/pdf/recombination-difference-between-sexes-a-role-for-haploid-33esg5qjeg.pdf

Recombination Difference between Sexes: A Role for Haploid Selection

Roses hold high symbolic value and great cultural importance in different societies throughout human history. They are widely used as garden ornamental plants, as cut flowers, and for the production of essential oils for the perfume and cosmetic industries. Domestication of roses has a long and complex history, and the rose species have been hybridized across vast geographic areas such as Europe, Asia, and the Middle East. The domestication processes selected several flower characters affecting floral quality, such as recurrent flowering, double flowers, petal colours, and fragrance. The molecular and genetic events that determine some of these flower characters cannot be studied using model species such as Arabidopsis thaliana, or at least only in a limited manner. In this review, we comment on the recent development of genetic, genomic, and transcriptomic tools for roses, and then focus on recent advances that have helped unravel the molecular mechanisms underlying several rose floral traits.

/pdf/genetics-and-genomics-of-flower-initiation-and-development-20n9x744qa.pdf

Genetics and genomics of flower initiation and development in roses.

In plants the switch from vegetative growth to flowering involves a major transition in the development of the shoot apex. This transition can occur once, in annual species, or repeatedly, in perennial plants. In annuals, flowering is associated with senescence and death of the whole plant, whereas perennials flower in consecutive years and maintain vegetative development after flowering. The perennial life strategy depends on differential behavior of meristems on a single plant so that some remain in the vegetative state while others undergo the floral transition. A. thaliana provides a powerful model system for understanding the mechanisms of flowering in annuals. Here we review the events that occur in the meristem of A. thaliana during the floral transition and compare these with our understanding of flowering in perennial systems.

Comparative analysis of flowering in annual and perennial plants.

The genus Rosa (roses) comprises approximately 190 shrub species distributed widely throughout the temperate and subtropical habitats of the northern hemisphere. Despite numerous recent studies examining phylogenetic relationships in the genus, relationships remain obscure due to problems such as poor identification of garden specimens, hybridization in nature and in the garden, and low levels of chloroplast and nuclear genome variation. Phylogenetic analyses of non-coding chloroplast sequences from the trnL-F region and psbA-trnH intergenic spacer for 70 taxa show slightly more variation than previous analyses of the genus. Bayesian and parsimony analyses suggest that subg. Rosa can be divided into two large clades, each with low internal resolution. One comprises species from sections Carolinae, Cinnamomeae and Pimpinellifoliae p.p., whilst the other consists of all of the remaining sections of subg. Rosa (Banksianae p.p., Bracteatae, Caninae, Indicae, Laevigatae, Rosa, Synstylae and Pimpinellifoliae p.p.). A fairly complete sampling of field-collected North American taxa has been incorporated in this analysis. Analyses indicate that migration into North America occurred at least twice within this primarily Old World genus. Most North American taxa, except R. setigera and R. minutifolia, fall into a single clade that includes Asian and European taxa. Analyses also are consistent with the notion that cultivated commercial roses have a relatively narrow genetic background. Six of the seven primary taxa believed to be involved in the creation of domesticated roses are found within the same large clade that mostly includes Asian and European taxa.

/pdf/phylogenetic-relationships-in-the-genus-rosa-new-evidence-59mgl4crx6.pdf

Phylogenetic Relationships in the Genus Rosa: New Evidence from Chloroplast DNA Sequences and an Appraisal of Current Knowledge

We have constructed the first integrated consensus map (ICM) for rose, based on the information of four diploid populations and more than 1,000 initial markers. The single population maps are linked via 59 bridge markers, on average 8.4 per linkage group (LG). The integrated map comprises 597 markers, 206 of which are sequence-based, distributed over a length of 530 cM on seven LGs. By using a larger effective population size and therefore higher marker density, the marker order in the ICM is more reliable than in the single population maps. This is supported by a more even marker distribution and a decrease in gap sizes in the consensus map as compared to the single population maps. This unified map establishes a standard nomenclature for rose LGs, and presents the location of important ornamental traits, such as self-incompatibility, black spot resistance (Rdr1), scent production and recurrent blooming. In total, the consensus map includes locations for 10 phenotypic single loci, QTLs for 7 different traits and 51 ESTs or gene-based molecular markers. This consensus map combines for the first time the information for traits with high relevance for rose variety development. It will serve as a tool for selective breeding and marker assisted selection. It will benefit future efforts of the rose community to sequence the whole rose genome and will be useful for synteny studies in the Rosaceae family and especially in the section Rosoideae.

Towards a unified genetic map for diploid roses

A segregating population of 91 hybrids issued from a cross between a dihaploid rose, derived from the haploidisation of a modern cultivar, and a diploid species was used to construct linkage maps of the parental genomes. As in other recent genetic studies in Rosa, AFLPs were used as molecular markers. Two segregating qualitative traits, recurrent blooming and double corolla, already known to be inherited as single recessive and dominant genes, respectively, were recorded in the mapping population. A quantitative trait, thorn density of the shoots, was also evaluated in this population. Sixty eight and 108 AFLP markers located on 8 and 6 linkage groups could be analysed in the female and male parent, respectively. The two recorded qualitative phenotypic markers were mapped as well as the quantitative one, after having performed QTL analyses on the parental maps in the latter case. It appears that thorn quantity is controlled by a major and a minor QTL which are located on the same linkage group at 36.5 and 3.2 cM from the single seasonal-blooming gene, respectively.

Mapping of qualitative and quantitative phenotypic traits in Rosa using AFLP markers.

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Mapping of qualitative and quantitative phenotypic traits in Rosa using AFLP markers.