No abstract

Polyploidy is commonly thought to be associated with the domestication process because of its concurrence with agriculturally favourable traits and because it is widespread among the major plant crops(1-4). Furthermore, the genetic consequences of polyploidy(5-7) might have increased the adaptive plasticity of those plants, enabling successful domestication(6-8). Nevertheless, a detailed phylogenetic analysis regarding the association of polyploidy with the domestication process, and the temporal order of these distinct events, has been lacking(3). Here, we have gathered a comprehensive data set including dozens of genera, each containing one or more major crop species and for which sufficient sequence and chromosome number data exist. Using probabilistic inference of ploidy levels conducted within a phylogenetic framework, we have examined the incidence of polyploidization events within each genus. We found that domesticated plants have gone through more polyploidy events than their wild relatives, with monocots exhibiting the most profound difference: 54% of the crops are polyploids versus 40% of the wild species. We then examined whether the preponderance of polyploidy among crop species is the result of two, non-mutually-exclusive hypotheses: (1) polyploidy followed by domestication, and (2) domestication followed by polyploidy. We found support for the first hypothesis, whereby polyploid species were more likely to be domesticated than their wild relatives, suggesting that the genetic consequences of polyploidy have conferred genetic preconditions for successful domestication on many of these plants.

Background: Cultivated watermelon form large fruits that are highly variable in size, shape, color, and content, yet have extremely narrow genetic diversity. Whereas a plethora of genes involved in cell wall metabolism, ethylene biosynthesis, fruit softening, and secondary metabolism during fruit development and ripening have been identified in other plant species, little is known of the genes involved in these processes in watermelon. A microarray and quantitative Real-Time PCR-based study was conducted in watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus] in order to elucidate the flow of events associated with fruit development and ripening in this species. RNA from three different maturation stages of watermelon fruits, as well as leaf, were collected from field grown plants during three consecutive years, and analyzed for gene expression using high-density photolithography microarrays and quantitative PCR.

Numerous studies of emerging species have identified genomic “islands” of elevated differentiation against a background of relative homogeneity. The causes of these islands remain unclear, however, with some signs pointing toward “speciation genes” that locally restrict gene flow and others suggesting selective sweeps that have occurred within nascent species after speciation. Here, we examine this question through the lens of genome sequence data for five species of southern capuchino seedeaters, finch-like birds from South America that have undergone a species radiation during the last ∼50,000 generations. By applying newly developed statistical methods for ancestral recombination graph inference and machine-learning methods for the prediction of selective sweeps, we show that previously identified islands of differentiation in these birds appear to be generally associated with relatively recent, species-specific selective sweeps, most of which are predicted to be soft sweeps acting on standing genetic variation. Many of these sweeps coincide with genes associated with melanin-based variation in plumage, suggesting a prominent role for sexual selection. At the same time, a few loci also exhibit indications of possible selection against gene flow. These observations shed light on the complex manner in which natural selection shapes genome sequences during speciation.

The flowering pattern of watermelon species (Citrullus spp.) is either monoecious or andromonoecious. Ethylene is known to play a critical role in floral sex determination of cucurbit species. In contrast to its feminizing effect in cucumber and melon, in watermelon ethylene promotes male flower development. In cucumber, the rate-limiting enzyme of ethylene biosynthesis, 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS), regulates unisexual flower development. To investigate the role of ethylene in flower development, we isolated four genomic sequences of ACS from watermelon (CitACS1-4). Both CitACS1 and CitACS3 are expressed in floral tissue. CitACS1 is also expressed in vegetative tissue and it may be involved in cell growth processes. Expression of CitACS1 is up-regulated by exogenous treatment with auxin, gibberellin or ACC, the immediate precursor of ethylene. No discernible differential floral sex-dependent expression pattern was observed for this gene. The CitACS3 gene is expressed in open flowers and in young staminate floral buds (male or hermaphrodite), but not in female flowers. CitACS3 is also up-regulated by ACC, and is likely to be involved in ethylene-regulated anther development. The expression of CitACS2 was not detected in vegetative or reproductive organs but was up-regulated by auxin. CitACS4 transcript was not detected under our experimental conditions. Restriction fragment length polymorphism (RFLP) and sequence tagged site (STS) marker analyses of the CitACS genes showed polymorphism among and within the different Citrullus groups, including watermelon cultivars, Citrullus lanatus var. lanatus, the central subspecies Citrullus lanatus var. citroides, and the desert species Citrullus colocynthis (L).