The sexual expression of watermelon plants is the result of the distribution and occurrence of male, female, bisexual and hermaphrodite flowers on the main and secondary stems. Plants can be monoecious (producing male and female flowers), andromonoecious (producing male and hermaphrodite flowers), or partially andromonoecious (producing male, female, bisexual, and hermaphrodite flowers) within the same plant. Sex determination of individual floral buds and the distribution of the different flower types on the plant, are both controlled by ethylene. A single missense mutation in the ethylene biosynthesis gene CitACS4, is able to promote the conversion of female into hermaphrodite flowers, and therefore of monoecy (genotype MM) into partial andromonoecy (genotype Mm) or andromonoecy (genotype mm). We phenotyped and genotyped, for the M/m locus, a panel of 207 C. lanatus accessions, including five inbreds and hybrids, and found several accessions that were repeatedly phenotyped as PA (partially andromonoecious) in several locations and different years, despite being MM. A cosegregation analysis between a SNV in CitACS4 and the PA phenotype, demonstrated that the occurrence of bisexual and hermaphrodite flowers in a PA line is not dependent on CitACS4, but conferred by an unlinked recessive gene which we called pa. Two different approaches were performed to map the pa gene in the genome of C. lanatus: bulk segregant analysis sequencing (BSA-seq) and genome wide association analysis studies (GWAS). The BSA-seq study was performed using two contrasting bulks, the monoecious M-bulk and the partially andromonoecious PA-bulk, each one generated by pooling DNA from 20 F2 plants. For GWAS, 122 accessions from USDA gene bank, already re-sequenced by genotyping by sequencing (GBS), were used. The combination of the two approaches indicates that pa maps onto a genomic region expanding across 32.24–36.44 Mb in chromosome 1 of watermelon. Fine mapping narrowed down the pa locus to a 867 Kb genomic region containing 101 genes. A number of candidate genes were selected, not only for their function in ethylene biosynthesis and signalling as well as their role in flower development and sex determination, but also by the impact of the SNPs and indels differentially detected in the two sequenced bulks.

Sex expression is an important trait in watermelon (Citrullus lanatus), in which monoecious, andromonoecious, and trimonoecious forms are present. The andromonoecious trait is highly undesirable in watermelon breeding programs because it would require emasculation during hybrid development. In contrast to other cucurbits such as melon (Cucumis melo) and cucumber (Cucumis sativus) in which the genes involved in sex expression are well described and have been cloned, only the inheritance of the a gene associated with the andromonoecious trait has been described in watermelon. We used an intersubspecific C. lanatus var. lanatus · C. lanatus var. citroides F2 population to map quantitative trait loci (QTL) associated with percent male (%M), percent female (%F), percent hermaphrodite (%HM), and percent female of pistillate [%F/P (female D hermaphrodite)] flowers. Four chromosomal regions were identified that were associated with sex expression in watermelon. Major QTL for %F, %HM, and %F/P were colocalized on linkage group 11A and explained 31.3% to 37.7% of the phenotypic variation observed for the three traits. These QTL are suggested to be the location of the a gene in watermelon. Markers linked to two of the four QTL identified were located within 1 Mb of a 1-aminocyclopropane-1-carboxylic acid synthase (ACS) gene on the watermelon draft genome. ACS genes play a key role in sex expression in melon and cucumber and these genes are therefore candidate genes for further studies to elucidate this trait in another economically important cucurbit crop. The QTL and candidate genes identified in the present study lay the foundation for marker-assisted selection for sex expression traits in watermelon. Watermelon is an economically important member of the Cucurbitaceae family. Sex expression of members of the family is diverse (Grumet and Taft, 2012) and in watermelon, monoecious (separate male and female flowers on the same plant), andromonoecious (separate male and hermaphrodite flowers on the same plant), and trimonoecious (separate male, female, and hermaphrodite flowers on the same plant) forms are found (Ferreira et al., 2002; Maynard, 2001; Robinson and Decker-Walters, 1997; Rosa, 1928). Commercial watermelon cultivars (Citrullus lanatus var. lanatus) express predominantly monoecious flowering types, whereas an andromonoecious flowering habit is common in the wild citron types (C. lanatus var. citroides). Sex expression is important for watermelon breeders, because the majority of commercially grown cultivars are F1 hybrids. Andromonoecious forms are highly undesirable because hermaphroditic flowers require emasculation before cross-pollination. In addition to the andromonoecious trait, the ratio of staminate:pistillate flowers is also of interest to breeders. Commercial watermelon cultivars usually have an approximate ratio of 7:1 staminate:pistillate flowers (Wehner, 2008), but this ratio can vary greatly. For the production of F1 hybrids, dioecy (separate male and female plants) are desirable, but this has not been observed in watermelon (Rudich and Zamski, 1985; Salman-Minkov et al., 2008). A gene (a) controlling the andromonoecious trait in watermelon was described in 1928 by Rosa (1928), but since then, little further research has been conducted to elucidate the trait (Poole and Grimball, 1945; Rudich and Zamski, 1985; SalmanMinkov et al., 2008). In contrast, the inheritance and control of sex expression in melon and cucumber, two other important cucurbits, has been studied in great detail. Briefly, in both melon and cucumber, sex determination is largely controlled by two genes; the A (andromonoecious) and G (gynoecious) genes in melon and the F (female) and M (monoecious) genes in cucumber (Grumet and Taft, 2012; Perl-Treves, 1999). In addition to these genes, modifier genes affecting sex expression have been described in these species and it has been suggested that at least four genes contribute to sex expression (Grumet

Monoecious and andromonoecious cultivars of watermelon are characterised by the production of male and female flower or male and hermaphrodite flowers, respectively. The segregation analysis in the offspring of crosses between monoecious and andromonoecious lines has demonstrated that this trait is controlled by a single gene pair, being the monoecious allele M semi-dominant to the andromonoecious allele A. The two studied F1 hybrids (MA) had a predominantly monoecious phenotype since both produced not only female flowers, but also bisexual flowers with incomplete stamens, and hermaphrodite fl

Mature fruit can be fleshy or dry. Fleshy fruit include the familiar berries, peaches, apples, grapes, and tomatoes. Rice, wheat, and nuts are examples of dry fruit. Another subtle distinction is that not all fruits are derived from just the ovary. For instance, strawberries are derived from the ovary as well as the receptacle, and apples are formed from the ovary and the pericarp, or hypanthium. Some fruits are derived from separate ovaries in a single flower, such as the raspberry. Other fruits, such as the pineapple, form from clusters of flowers. Additionally, some fruits, like watermelon and orange, have rinds. Regardless of how they are formed, fruits are an agent of seed dispersal. The variety of shapes and characteristics reflect the mode of dispersal. Wind carries the light dry fruits of trees and dandelions. Water transports floating coconuts. Some fruits attract herbivores with their color or scent, or as food. Once eaten, tough, undigested seeds are dispersed through the herbivore’s feces (endozoochory). Other fruits have burrs and hooks to cling to fur and hitch rides on animals (epizoochory). Angiosperms are classified in a single phylum: the Anthophyta.

melons and cucumbers, present one very curious peculiarity. In all these plants, the sexes are distinct ; and , in most of them, the male and female flowers