Comprehensive profiling of endogenous phytohormones and expression analysis of 1-aminocyclopropane-1-carboxylic acid synthase gene family during fruit development and ripening in octoploid strawberry (Fragaria× ananassa).

The non-climacteric octoploid strawberry (Fragaria × ananassa Duchesne ex Rozier) was used as a model to study its regulation during fruit ripening. High performance liquid chromatography electrospray tandem-mass spectrometry (HPLC-ESI-MS/MS) was employed to profile 28 different endogenous phytohormones in strawberry. These include auxins, cytokinins (CKs), abscisic acid (ABA), ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC), jasmonates, and phenolic compounds salicylic acid (SA), benzoic acid (BzA) and phenylacetic acid (PAA) together with their various metabolic forms that have remained largely unexplored thus far. ABA, ACC and CK N6-(Δ2-isopentenyl)adenine (iP) were found to be associated with ripening while ABA catabolites 9-hydroxy-ABA and phaseic acid mimicked the pattern of climacteric decline at the turning phase of strawberry ripening. The content of other CK forms except iP decreased as fruit ripened, as also that of auxins indole-3-acetic acid (IAA) and oxo-IAA, and of jasmonates. Data presented here also suggest that both the transition and progression of strawberry fruit ripening are associated with N6-(Δ2-isopentenyl)adenosine-5'-monophosphate (iPRMP) → N6-(Δ2-isopentenyl)adenosine (iPR) → iP as the preferred CK metabolic pathway. In contrast, the ethylene precursor ACC was present at higher levels, with its abundance increasing from the onset of ripening to the red ripe stage. Further investigation of ripening-specific ACC accumulation revealed the presence of a large ACC synthase (ACS) encoding gene family in octoploid strawberry that was previously unknown. Seventeen ACS genes were found differentially expressed in fruit tissues, while six of them showed induced expression during strawberry fruit ripening. These data suggest a possible role(s) of ACC, ABA, and iP in strawberry fruit ripening. These data add new dimension to the existing knowledge of the interplay of different endogenous phytohormones in octoploid strawberry, paving the way for further investigation of their individual role(s) in fruit ripening.

Genetic architecture of sexual dimorphism in a subdioecious plant with a proto-sex chromosome.

The rise of sexual dimorphism is thought to coincide with the evolution of sex chromosomes. Yet because sex chromosomes in many species are ancient, we lack empirical evidence of the earliest stages of this transition. We use QTL analysis to examine the genetic architecture of sexual dimorphism in subdioecious octoploid Fragaria virginiana. We demonstrate that the region housing the male-function locus controls the majority of quantitative variation in proportion fruit set, confirming the existence of a proto-sex chromosome, and houses major QTL for eight additional sexually dimorphic traits, consistent with theory and data from animals and plants with more advanced sex chromosomes. We also detected autosomal QTL, demonstrating contributions to phenotypic variation in sexually dimorphic traits outside the sex-determining region. Moreover, for proportion seed set we found significant epistatic interactions between autosomal QTL and the male-function locus, indicating sex-limited QTL. We identified linked QTL reflecting trade-offs between male and female traits expected from theory and positive integration of male traits. These findings indicate the potential for the evolution of greater sexual dimorphism. Involvement of linkage groups homeologous to the proto-sex chromosome in these correlations reflects the polyploid origin of F. virginiana and raises the possibility that chromosomes in this homeologous group were predisposed to become the sex chromosome.

Comparative mapping reveals autosomal origin of sex chromosome in octoploid Fragaria virginiana.

Recent evolution of separate sexes in flowering plants provides unparalleled opportunities for understanding the early stages of sex chromosome evolution, including their origin from autosomes. Moreover, the transition from combined to separate sexes can be associated with speciation via polyploidization in angiosperms, suggesting that genome doubling/merger may facilitate sterility mutations required for sex chromosome formation. To gain insight into the origin of sex chromosomes in a polyploid plant, we doubled the simple sequence repeat (SSR) density and increased genome coverage in a genetic map of octoploid Fragaria virginiana, a species purported to have a "proto-sex" chromosome, where limited recombination occurs between 2 linked "loci" carrying the male- and female-sterility mutations. Incorporation of almost 3 times the number of SSR markers into the current map facilitated complete characterization of the F. virginiana proto-sex chromosome, revealing its largely autosomal nature and the location of the sex-determining region toward the distal end. Furthermore, extensive synteny between our genetic map and a map involving diploid hermaphroditic congeners allowed assignment of linkage groups to homeologous groups, identification of the proto-sex chromosome's autosomal homoeolog, and detection of a putative rearrangement near the sex-determining region. Fine mapping and additional comparative work will shed light on the intriguing possibility that rearrangements during polyploidization were involved in the evolution of sex chromosomes in Fragaria.