Meiotic gene evolution: can you teach a new dog new tricks?

Meiosis, the basis of sex, evolved through iterative gene duplications. To understand whether subsequent duplications have further enriched the core meiotic "tool-kit," we investigated the fate of meiotic gene duplicates following whole genome duplication (WGD), a common occurrence in eukaryotes. We show that meiotic genes return to a single copy more rapidly than genome-wide average in angiosperms, one of the lineages in which WGD is most vividly exemplified. The rate at which duplicates are lost decreases through time, a tendency that is also observed genome-wide and may thus prove to be a general trend post-WGD. The sharpest decline is observed for the subset of genes mediating meiotic recombination; however, we found no evidence that the presence of these duplicates is counterselected in two recent polyploid crops selected for fertility. We therefore propose that their loss is passive, highlighting how quickly WGDs are resolved in the absence of selective duplicate retention.

The TFL1 homologue KSN is a regulator of continuous flowering in rose and strawberry.

Flowering is a key event in plant life, and is finely tuned by environmental and endogenous signals to adapt to different environments. In horticulture, continuous flowering (CF) is a popular trait introduced in a wide range of cultivated varieties. It played an essential role in the tremendous success of modern roses and woodland strawberries in gardens. CF genotypes flower during all favourable seasons, whereas once-flowering (OF) genotypes only flower in spring. Here we show that in rose and strawberry continuous flowering is controlled by orthologous genes of the TERMINAL FLOWER 1 (TFL1) family. In rose, six independent pairs of CF/OF mutants differ in the presence of a retrotransposon in the second intron of the TFL1 homologue. Because of an insertion of the retrotransposon, transcription of the gene is blocked in CF roses and the absence of the floral repressor provokes continuous blooming. In OF-climbing mutants, the retrotransposon has recombined to give an allele bearing only the long terminal repeat element, thus restoring a functional allele. In OF roses, seasonal regulation of the TFL1 homologue may explain the seasonal flowering, with low expression in spring to allow the first bloom. In woodland strawberry, Fragaria vesca, a 2-bp deletion in the coding region of the TFL1 homologue introduces a frame shift and is responsible for CF behaviour. A diversity analysis has revealed that this deletion is always associated with the CF phenotype. Our results demonstrate a new role of TFL1 in perennial plants in maintaining vegetative growth and modifying flowering seasonality.

Variation in crossover rates across a 3-Mb contig of bread wheat (Triticum aestivum) reveals the presence of a meiotic recombination hotspot.

In bread wheat (Triticum aestivum L.), initial studies using deletion lines indicated that crossover (CO) events occur mainly in the telomeric regions of the chromosomes with a possible correlation with the presence of genes. However, little is known about the distribution of COs at the sequence level. To investigate this, we studied in detail the pattern of COs along a contig of 3.110 Mb using two F2 segregating populations (Chinese Spring × Renan (F2-CsRe) and Chinese Spring × Courtot (F2-CsCt)) each containing ~2,000 individuals. The availability of the sequence of the contig from Cs enabled the development of 318 markers among which 23 co-dominant polymorphic markers (11 SSRs and 12 SNPs) were selected for CO distribution analyses. The distribution of CO events was not homogeneous throughout the contig, ranging from 0.05 to 2.77 cM/Mb, but was conserved between the two populations despite very different contig recombination rate averages (0.82 cM/Mb in F2-CsRe vs 0.35 cM/Mb in F2-CsCt). The CO frequency was correlated with the percentage of coding sequence in Cs and with the polymorphism rate between Cs and Re or Ct in both populations, indicating an impact of these two factors on CO distribution. At a finer scale, COs were found in a region covering 2.38 kb, spanning a gene coding for a glycosyl transferase (Hga3), suggesting the presence of a CO hotspot. A non-crossover event covering at least 453 bp was also identified in the same interval. From these results, we can conclude that gene content could be one of the factors driving recombination in bread wheat.

Genetic Structure of a Worldwide Germplasm Collection of Prunus armeniaca L. Reveals Three Major Diffusion Routes for Varieties Coming From the Species' Center of Origin.

The characterization of the largest worldwide representative data set of apricot (Prunus armeniaca L.) germplasm was performed using molecular markers. Genetic diversity and structure of the cultivated apricot genetic resources were analyzed to decipher the history of diffusion of this species around the world. A common set of 25 microsatellite markers was used for genotyping a total of 890 apricot accessions in different collections from the center of origin to the more recent regions of apricot culture. Using a Bayesian model-based clustering approach, the apricot genotypes can be structured into five different genetic clusters (FST = 0.174), correlated with the geographical regions of origin of the accessions. Accessions from China and Central Asia were clustered together and exhibited the highest levels of diversity, confirming an origin in this region. A loss of genetic diversity was observed from the center of origin to both western and eastern zones of recent apricot culture. Altogether, our results revealed that apricot spread from China and Central Asia, defined as the center of origin, following three major diffusion routes with a decreasing gradient of genetic variation in each geographical group. The identification of specific alleles outside the center of origin confirmed the existence of different secondary apricot diversification centers. The present work provides more understanding of the worldwide history of apricot species diffusion as well as the field of conservation of the available genetic resources. Data have been used to define an apricot core collection based on molecular marker diversity which will be useful for further identification of genomic regions associated with commercially important horticultural traits through genome-wide association studies to sustain apricot breeding programs.

A survey of flowering genes reveals the role of gibberellins in floral control in rose.

Exhaustive studies on flowering control in annual plants have provided a framework for exploring this process in other plant species, especially in perennials for which little molecular data are currently available. Rose is a woody perennial plant with a particular flowering strategy--recurrent blooming, which is controlled by a recessive locus (RB). Gibberellins (GA) inhibit flowering only in non-recurrent roses. Moreover, the GA content varies during the flowering process and between recurrent and non-recurrent rose. Only a few rose genes potentially involved in flowering have been described, i.e. homologues of ABC model genes and floral genes from EST screening. In this study, we gained new information on the molecular basis of rose flowering: date of flowering and recurrent blooming. Based on a candidate gene strategy, we isolated genes that have similarities with genes known to be involved in floral control in Arabidopsis (GA pathway, floral repressors and integrators). Candidate genes were mapped on a segregating population, gene expression was studied in different organs and transcript abundance was monitored in growing shoot apices. Twenty-five genes were studied. RoFT, RoAP1 and RoLFY are proposed to be good floral markers. RoSPY and RB co-localized in our segregating population. GA metabolism genes were found to be regulated during floral transition. Furthermore, GA signalling genes were differentially regulated between a non-recurrent rose and its recurrent mutant. We propose that flowering gene networks are conserved between Arabidopsis and rose. The GA pathway appears to be a key regulator of flowering in rose. We postulate that GA metabolism is involved in floral initiation and GA signalling might be responsible for the recurrent flowering character.

Genomic approach to study floral development genes in Rosa sp.

Cultivated for centuries, the varieties of rose have been selected based on a number of flower traits. Understanding the genetic and molecular basis that contributes to these traits will impact on future improvements for this economically important ornamental plant. In this study, we used scanning electron microscopy and sections of meristems and flowers to establish a precise morphological calendar from early rose flower development stages to senescing flowers. Global gene expression was investigated from floral meristem initiation up to flower senescence in three rose genotypes exhibiting contrasted floral traits including continuous versus once flowering and simple versus double flower architecture, using a newly developed Affymetrix microarray (Rosa1_Affyarray) tool containing sequences representing 4765 unigenes expressed during flower development. Data analyses permitted the identification of genes associated with floral transition, floral organs initiation up to flower senescence. Quantitative real time PCR analyses validated the mRNA accumulation changes observed in microarray hybridizations for a selection of 24 genes expressed at either high or low levels. Our data describe the early flower development stages in Rosa sp, the production of a rose microarray and demonstrate its usefulness and reliability to study gene expression during extensive development phases, from the vegetative meristem to the senescent flower.