Catherine Dogimont

A transposon-induced epigenetic change leads to sex determination in melon

Sex determination in plants leads to the development of unisexual flowers from an originally bisexual floral meristem. This mechanism results in the enhancement of outcrossing and promotes genetic variability, the consequences of which are advantageous to the evolution of a species. In melon, sexual forms are controlled by identity of the alleles at the andromonoecious (a) and gynoecious (g) loci. We previously showed that the a gene encodes an ethylene biosynthesis enzyme, CmACS-7, that represses stamen development in female flowers. Here we show that the transition from male to female flowers in gynoecious lines results from epigenetic changes in the promoter of a transcription factor, CmWIP1. This natural and heritable epigenetic change resulted from the insertion of a transposon, which is required for initiation and maintenance of the spreading of DNA methylation to the CmWIP1 promoter. Expression of CmWIP1 leads to carpel abortion, resulting in the development of unisexual male flowers. Moreover, we show that CmWIP1 indirectly represses the expression of the andromonoecious gene, CmACS-7, to allow stamen development. Together our data indicate a model in which CmACS-7 and CmWIP1 interact to control the development of male, female and hermaphrodite flowers in melon.

A Conserved Mutation in an Ethylene Biosynthesis Enzyme Leads to Andromonoecy in Melons

Andromonoecy is a widespread sexual system in angiosperms characterized by plants carrying both male and bisexual flowers. In melon, this sexual form is controlled by the identity of the alleles at the andromonoecious (a) locus. Cloning of the a gene reveals that andromonoecy results from a mutation in the active site of 1-aminocyclopropane-1-carboxylic acid synthase. Expression of the active enzyme inhibits the development of the male organs and is not required for carpel development. A causal single-nucleotide polymorphism associated with andromonoecy was identified, which suggests that the a allele has been under recent positive selection and may be linked to the evolution of this sexual system.

A consensus linkage map for molecular markers and quantitative trait loci associated with economically important traits in melon (Cucumis melo L.).

BackgroundA number of molecular marker linkage maps have been developed for melon (Cucumis melo L.) over the last two decades. However, these maps were constructed using different marker sets, thus, making comparative analysis among maps difficult. In order to solve this problem, a consensus genetic map in melon was constructed using primarily highly transferable anchor markers that have broad potential use for mapping, synteny, and comparative quantitative trait loci (QTL) analysis, increasing breeding effectiveness and efficiency via marker-assisted selection (MAS).ResultsUnder the framework of the International Cucurbit Genomics Initiative (ICuGI, http://www.icugi.org), an integrated genetic map has been constructed by merging data from eight independent mapping experiments using a genetically diverse array of parental lines. The consensus map spans 1150 cM across the 12 melon linkage groups and is composed of 1592 markers (640 SSRs, 330 SNPs, 252 AFLPs, 239 RFLPs, 89 RAPDs, 15 IMAs, 16 indels and 11 morphological traits) with a mean marker density of 0.72 cM/marker. One hundred and ninety-six of these markers (157 SSRs, 32 SNPs, 6 indels and 1 RAPD) were newly developed, mapped or provided by industry representatives as released markers, including 27 SNPs and 5 indels from genes involved in the organic acid metabolism and transport, and 58 EST-SSRs. Additionally, 85 of 822 SSR markers contributed by Syngenta Seeds were included in the integrated map. In addition, 370 QTL controlling 62 traits from 18 previously reported mapping experiments using genetically diverse parental genotypes were also integrated into the consensus map. Some QTL associated with economically important traits detected in separate studies mapped to similar genomic positions. For example, independently identified QTL controlling fruit shape were mapped on similar genomic positions, suggesting that such QTL are possibly responsible for the phenotypic variability observed for this trait in a broad array of melon germplasm.ConclusionsEven though relatively unsaturated genetic maps in a diverse set of melon market types have been published, the integrated saturated map presented herein should be considered the initial reference map for melon. Most of the mapped markers contained in the reference map are polymorphic in diverse collection of germplasm, and thus are potentially transferrable to a broad array of genetic experimentation (e.g., integration of physical and genetic maps, colinearity analysis, map-based gene cloning, epistasis dissection, and marker-assisted selection).

Host plant resistance to aphids in cultivated crops: Genetic and molecular bases, and interactions with aphid populations

Host plant resistance is an efficient and environmentally friendly means of controlling insects, including aphids, but resistant-breaking biotypes have occurred in several plant-aphid systems. Our review of the genetic and molecular bases of aphid resistance in crop species emphasizes the limited number of aphid resistance genes and alleles. Inheritance of aphid resistance may be monogenic (dominant or recessive genes) or polygenic. Two dominant, aphid resistance genes have been isolated to date. They both encode NBS-LRR proteins involved in the specific recognition of aphids. Strategies to ensure aphid resistance effectiveness and durability are discussed. Innovative research activities are proposed.

Molecular and Genetic Characterization of a Non-Climacteric Phenotype in Melon Reveals Two Loci Conferring Altered Ethylene Response in Fruit

Fruit ripening and abscission are associated with an ethylene burst in several melon (Cucumis melo) genotypes. In cantaloupe as in other climacteric fruit, exogenous ethylene can prematurely induce abscission, ethylene production, and ripening. Melon genotypes without fruit abscission or without ethylene burst also exist and are, therefore, non-climacteric. In the nonabscising melon fruit PI 161375, exogenous ethylene failed to stimulate abscission, loss of firmness, ethylene production, and expression of all target genes tested. However, the PI 161375 etiolated seedlings displayed the usual ethylene-induced triple response. Genetic analysis on a population of recombinant cantaloupe Charentais × PI 161375 inbred lines in segregation for fruit abscission and ethylene production indicated that both characters are controlled by two independent loci, abscission layer(Al)-3 and Al-4. The non-climacteric phenotype in fruit tissues is attributable to ethylene insensitivity conferred by the recessive allelic forms from PI 161375. Five candidate genes (two ACO, two ACS, and ERS) that were localized on the melon genetic map did not exhibit colocalization with Al-3 orAl-4.

Resistance gene homologues in melon are linked to genetic loci conferring disease and pest resistance

Abstract.Genomic and cDNA fragments with homology to known disease resistance genes (RGH fragments) were cloned from Cucumis melo using degenerate-primer PCR. Fifteen homologues of the NBS-LRR gene family have been isolated. The NBS-LRR homologues show high divergence and, based on the partial NBS-fragment sequences, appear to include members of the two major subfamilies that have been described in dicot plants, one that possesses a TIR-protein element and one that lacks such a domain. Genomic organization of these sequences was explored by DNA gel-blot analysis, and conservation among other Cucurbitaceae was assessed. Two mapping populations that segregate for several disease and pest resistance loci were used to map the RGH probes onto the melon genetic map. Several NBS-LRR related sequences mapped to the vicinity of genetic loci that control resistance to papaya ringspot virus, Fusarium oxysporum race 1, F. oxysporum race 2 and to the insect pest Aphis gossypii. The utility of such markers for breeding resistant melon cultivars and for cloning the respective R-genes is discussed.

Engineering Melon Plants with Improved Fruit Shelf Life Using the TILLING Approach

Background Fruit ripening and softening are key traits that have an effect on food supply, fruit nutritional value and consequently, human health. Since ethylene induces ripening of climacteric fruit, it is one of the main targets to control fruit over ripening that leads to fruit softening and deterioration. The characterization of the ethylene pathway in Arabidopsis and tomato identified key genes that control fruit ripening. Methodology/Principal Findings To engineer melon fruit with improved shelf-life, we conducted a translational research experiment. We set up a TILLING platform in a monoecious and climacteric melon line, cloned genes that control ethylene production and screened for induced mutations that lead to fruits with enhanced shelf life. Two missense mutations, L124F and G194D, of the ethylene biosynthetic enzyme, ACC oxidase 1, were identified and the mutant plants were characterized with respect to fruit maturation. The L124F mutation is a conservative mutation occurring away from the enzyme active site and thus was predicted to not affect ethylene production and thus fruit ripening. In contrast, G194D modification occurs in a highly conserved amino acid position predicted, by crystallographic analysis, to affect the enzymatic activity. Phenotypic analysis of the G194D mutant fruit showed complete delayed ripening and yellowing with improved shelf life and, as predicted, the L124F mutation did not have an effect. Conclusions/Significance We constructed a mutant collection of 4023 melon M2 families. Based on the TILLING of 11 genes, we calculated the overall mutation rate of one mutation every 573 kb and identified 8 alleles per tilled kilobase. We also identified a TILLING mutant with enhanced fruit shelf life. This work demonstrates the effectiveness of TILLING as a reverse genetics tool to improve crop species. As cucurbits are model species in different areas of plant biology, we anticipate that the developed tool will be widely exploited by the scientific community.

Genetic control of fruit shape acts prior to anthesis in melon ( Cucumis melo L.).

Abstract. Genetic control of fruit shape in Cucumis melo was studied using QTL analysis in two Recombinant Inbred (RI) populations consisting of 163 and 63 individuals, respectively, obtained by crossing the same round-fruited parent with two different elongated-fruit lines. Fruit shape is mainly explained by fruit length in these two populations. Most QTLs for fruit shape and ovary shape detected were found to co-segregate, thus demonstrating early control of fruit shape during ovary development. A high level of correlation between fruit shape and ovary shape was also found in 14 unrelated genetic lines, a finding which suggests that control of fruit shape by gene(s) active early in the ovary is a general feature in C. melo. Two major flower genes, a (monoecious) and p (pentamerous), were shown to have major effects on fruit shape. Major tightly linked QTLs for fruit and ovary shape were found close to the a and p genes, probably reflecting their pleiotropic effect on fruit shape. Moreover, one of the two QTLs detected in the Védrantais × PI 414723 population was also found in the Védrantais × PI 161375 population. Variation of fruit shape in melon could be due to variations having quantitative effects on a large set of genes that are probably involved in ovary development.

Genetic analysis of broad spectrum resistance to potyviruses using doubled haploid lines of pepper (Capsicum annuum L.)

SummaryGenetic analysis of resistance to PVY in androgenetic doubled haploid lines, F1, F2 and backcross progenies of the Mexican pepper line, CM 334 (Capsicum annuum L.), was performed. Three reaction types were observed when seedlings were inoculated with several PVY strains of different pathotypes and with an American PeMV strain. Resistant genotypes never showed systemic symptoms although some individuals sporadically developed necrotic local lesions on inoculated cotyledons. Susceptible genotypes exhibited either a typical systemic mosaic or a systemic necrosis that caused the death of the inoculated seedlings. Segregation analyses indicated that resistance to pepper potyviruses in CM 334 is conferred by two genes. The first one, tentatively named Pr4, is dominant and confers the resistance to all now known pathotypes of PVY and to PeMV. The second one, tentatively named pr5, is recessive; it confers only the resistance to common strains of PVY. The systemic necrotic response is conferred by an independent dominant gene, tentatively named Pn1.

A cucurbit androecy gene reveals how unisexual flowers develop and dioecy emerges

How flowers separate males and females Most flowering plant families have bisexual flowers with both male and female function. However, most members of the Cucurbiticeae family, which includes melons, cucumbers, and gourds, have unisexual flowers. To understand this difference in sex expression, Boualem et al. identified a cucumber gene expressed in the female flowers. Mutations in this gene were associated with solely male flowers. By integrating this finding into a sex determination model, the authors explain how unisexual flowers can coexist in the same plant. Science, this issue p. 688 A gene in melon and cucumber explains how separate-sex flowers can develop and coexist on the same plant. Understanding the evolution of sex determination in plants requires identifying the mechanisms underlying the transition from monoecious plants, where male and female flowers coexist, to unisexual individuals found in dioecious species. We show that in melon and cucumber, the androecy gene controls female flower development and encodes a limiting enzyme of ethylene biosynthesis, ACS11. ACS11 is expressed in phloem cells connected to flowers programmed to become female, and ACS11 loss-of-function mutants lead to male plants (androecy). CmACS11 represses the expression of the male promoting gene CmWIP1 to control the development and the coexistence of male and female flowers in monoecious species. Because monoecy can lead to dioecy, we show how a combination of alleles of CmACS11 and CmWIP1 can create artificial dioecy.