Sex-determining chromosomes and sexual dimorphism: insights from genetic mapping of sex expression in a natural hybrid Fragaria × ananassa subsp. cuneifolia

Abstract

We studied the natural hybrid (Fragaria  ananassa subsp. cuneifolia) between two sexually dimorphic octoploid strawberry species (Fragaria virginiana and Fragaria chiloensis) to gain insight into the dynamics of sex chromosomes and the genesis of sexual dimorphism. Male sterility is dominant in both the parental species and thus will be inherited maternally, but the chromosome that houses the sex-determining region differs. Thus, we asked whether (1) the cytotypic composition of hybrid populations represents one or both maternal species, (2) the sex-determining chromosome of the hybrid reflects the location of male sterility within the maternal donor species and (3) crosses from the hybrid species show less sexual dimorphism than the parental species. We found that F.  ananassa subsp. cuneifolia populations consisted of both parental cytotypes but one predominated within each population. Genetic linkage mapping of two crosses showed dominance of male sterility similar to the parental species, however, the map location of male sterility reflected the maternal donor in one cross, but not the other. Moreover, female function mapped to a single region in the first cross, but to two regions in the second cross. Aside from components of female function (fruit set and seed set), other traits that have been found to be significantly sexually dimorphic in the pure species were either not dimorphic or were dimorphic in the opposite direction to the parental species. These results suggest that hybrids experience some disruption of dimorphism in secondary sexual traits, as well as novel location and number of quantitative trait locus (QTL) affecting sex function.

Targeted Sequence Capture Provides Insight into Genome Structure and Genetics of Male Sterility in a Gynodioecious Diploid Strawberry,Fragaria vescassp.bracteata(Rosaceae)

“…Male sterility is dominant, as seen in other Fragaria species, but its chromosomal location is not syntenic with the sex-determining regions of octoploid Fragaria (Goldberg et al 2010; Spigler et al 2011; Govindarajulu et al 2013). There are three possible explanations for these observations.…”

“…Recent genetic mapping in the octoploid species and their natural hybrid confirms that male sterility is dominant to male fertility but also that it shows linkage to female fertility (Goldberg et al 2010; Spigler et al 2011; Govindarajulu et al 2013). Existing observations support conflicting hypotheses about the evolutionary origin of male sterility.…”

“…On one hand, the close relationship among several species displaying the rare trait of dominant male sterility suggests a single origin of the trait, with possible translocation (Goldberg et al 2010). On the other hand, the observation that male sterility maps to different genomic regions in the polyploid Fragaria species (Goldberg et al 2010; Spigler et al 2011; Govindarajulu et al 2013), as well as the existence of hermaphroditic F. iturupensis within an otherwise-sexually dimorphic clade (Njuguna et al 2013), suggest that male sterility may have evolved several times independently. Moreover, equivocal evidence for a fitness advantage sufficient to maintain females ( i.e.…”

Insights into the genetic architecture of sexual dimorphism from an interspecific cross between two diverging Silene (Caryophyllaceae) species

“…We applied data transformations to normalized data when needed and Bonferroni corrections to account for multiple testing. To visualize the degree of sexual dimorphism, we also estimated a sexual dimorphism index for both parental species, and the F1 and F2 generations (Govindarajulu et al, ; McDaniel, ; Spigler, Lewers, & Ashman, ) using the formula: ( Χ F − Χ M)/[( SE F + SE M)/2, where Χ and SE are the mean and standard error, respectively, and F and M stand for females and males, respectively. Positive values indicate that females are larger than males.…”

“…To date, only a handful of studies have investigated the genetic architecture of sexual dimorphism in plants (Delph et al, ; Govindarajulu et al, ; Spigler, Lewers, Main, & Ashman, ), and because there is a great diversity in sex determining systems in plants (Bachtrog et al, ; Charlesworth & Charlesworth, ), more studies are needed to understand the evolution of sexual dimorphism. Spigler et al () and Govindarajulu et al () have focused on the genetic architecture of sexual dimorphism and sex determination in the genus Fragaria , in which sex chromosomes have evolved recently and where females are the heterogametic (ZW) sex. These studies have given valuable information about the genetic basis of the transition from subdioecy to dioecy.…”