The X Chromosome Is Necessary for Somatic Development in the Dioecious <b><i>Silene latifolia</i></b>: Cytogenetic and Molecular Evidence and Sequencing of a Haploid Genome

Abstract

Silene latifolia (or white campion) possesses a well-established sex determination system with a dominant Y chromosome in males (the mammalian type). The heteromorphic sex chromosomes X and Y in S. latifolia largely stopped recombination; thus, we can expect a gradual genetic degeneration of the Y chromosome. It is well proven that neither diploid nor polyploid S. latifolia sporophytes can survive without at least one X, so the only life stage possessing the Y as the sole sex chromosome is the male gametophyte (pollen tube), while the female gametophyte seems to be X-dependent. Previous studies on anther-derived plants of this species showed that the obtained plants (largely haploid or dihaploid) were phenotypically and cytologically female. In this paper, we provide molecular evidence for the inviability of plants lacking the X chromosome. Using sex-specific PCR primers, we show that all plantlets and plants derived from anther cultures are female. In studying anther-derived diploid females by sequencing of X-linked markers, we demonstrate that these plants are really homozygous dihaploids. A haploid regenerant plant was sequenced (8× genome coverage) using Illumina technology. Genome data are disposable in the EMBL database as a standard for full genome and X chromosome assembly in this model species. Homozygous dihaploids were back-crossed with males to yield a progeny useful for the study of the evolution of the Y chromosome.

YY males of the dioecious plant Mercurialis annua are fully viable but produce largely infertile pollen

“…While it is possible that YY individuals suffer from mildly deleterious effects that we were unable to detect, or from effects on traits that we did not measure, the viability and phenotypic similarity between individuals with and without an X chromosome (or with a single or double dose of the Y) are in striking contrast with studies that show substantially poorer performance of YY males compared with their XY counterparts. For instance, Y‐chromosome degeneration has led to YY lethality in many animal species (Graves, ), as well as in the plants Rumex hastatulus (Smith, ), Silene latifolia (Westergaard, ; Janoušek et al ., ; Soukupova et al ., ; Veltsos & Delph, ), and Carica papaya (Liu et al ., ; Ming & Moore, ; Yu et al ., 2008a). These species probably have older sex chromosomes than M. annua, with a longer history of Y‐chromosome degeneration.…”

“…Here, WW hermaphrodites are viable but have substantially lower fitness than their ZW counterparts, pointing to recessive deleterious effects of W‐linked alleles (Sassaman & Weeks, ). In plants, YY individuals are nonviable in both Silene latifolia (Janoušek et al ., ; Soukupova et al ., ; Veltsos & Delph, ), which has highly divergent heteromorphic sex chromosomes (Krasovec et al ., ), and in Carica papaya , which has homomorphic sex chromosomes at the cytological level but shows XY divergence at the sequence level (Liu et al ., ; Yu et al ., 2008b). Viable YY males have been reported in Asparagus officinalis (Harkess et al ., ), Spinacia oleracea (Yamamoto et al ., ; Wadlington & Ming, ), Cannabis sativa (Peil et al ., ), Phoenix dactylifera and Actinidia chinensis (reviewed in Ming et al ., ).…”

The X chromosome is necessary for ovule production in Silene latifolia

“…latifolia . Our study adds to the body of evidence that YY plants are inviable based on artificial means (e.g., mutant Y chromosomes and disruption of methylation [1,20,21]). The departure from a 1:2 XX:XY sex ratio in the offspring and the low number of total ovules in our study allows us to conclude that the development of the female gametophyte requires an X chromosome.…”

“…These plants were produced by disrupting the methylation profile of male plants, and as above, only female offspring were produced from these XY individuals, suggesting that the X chromosome is required to produce ovules. Lastly, recent androgenesis experiments, in which pollen grains were regenerated into mature plants, produced only XX individuals that were found to be diploidized [21]. The lack of YY individuals from androgenesis again suggests that the Y is insufficient for normal plant development.…”