Isolation of genes from plant Y chromosomes

Identifying new sex-linked genes through BAC sequencing in the dioecious plant Silene latifolia

Background

Silene latifolia represents one of the best-studied plant sex chromosome systems. A new approach using RNA-seq data has recently identified hundreds of new sex-linked genes in this species. However, this approach is expected to miss genes that are either not expressed or are expressed at low levels in the tissue(s) used for RNA-seq. Therefore other independent approaches are needed to discover such sex-linked genes.

Results

Here we used 10 well-characterized S. latifolia sex-linked genes and their homologs in Silene vulgaris, a species without sex chromosomes, to screen BAC libraries of both species. We isolated and sequenced 4 Mb of BAC clones of S. latifolia X and Y and S. vulgaris genomic regions, which yielded 59 new sex-linked genes (with S. vulgaris homologs for some of them). We assembled sequences that we believe represent the tip of the Xq arm. These sequences are clearly not pseudoautosomal, so we infer that the S. latifolia X has a single pseudoautosomal region (PAR) on the Xp arm. The estimated mean gene density in X BACs is 2.2 times lower than that in S. vulgaris BACs, agreeing with the genome size difference between these species. Gene density was estimated to be extremely low in the Y BAC clones. We compared our BAC-located genes with the sex-linked genes identified in previous RNA-seq studies, and found that about half of them (those with low expression in flower buds) were not identified as sex-linked in previous RNA-seq studies. We compiled a set of ~70 validated X/Y genes and X-hemizygous genes (without Y copies) from the literature, and used these genes to show that X-hemizygous genes have a higher probability of being undetected by the RNA-seq approach, compared with X/Y genes; we used this to estimate that about 30 % of our BAC-located genes must be X-hemizygous. The estimate is similar when we use BAC-located genes that have S. vulgaris homologs, which excludes genes that were gained by the X chromosome.

Conclusions

Our BAC sequencing identified 59 new sex-linked genes, and our analysis of these BAC-located genes, in combination with RNA-seq data suggests that gene losses from the S. latifolia Y chromosome could be as high as 30 %, higher than previous estimates of 10-20 %.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1698-7) contains supplementary material, which is available to authorized users.

Plant Y chromosome degeneration is retarded by haploid purifying selection

Sex chromosomes evolved many times independently in many different organisms [1]. According to the currently accepted model, X and Y chromosomes evolve from a pair of autosomes via a series of inversions leading to stepwise expansion of a nonrecombining region on the Y chromosome (NRY) and the consequential degeneration of genes trapped in the NRY [2]. Our results suggest that plants represent an exception to this rule as a result of their unique life-cycle that includes alteration of diploid and haploid generations and widespread haploid expression of genes in plant gametophytes [3]. Using a new high-throughput approach, we identified over 400 new genes expressed from X and Y chromosomes in Silene latifolia, a plant that evolved sex chromosomes about 10 million years ago. Y-linked genes show faster accumulation of amino-acid replacements and loss of expression, compared to X-linked genes. These degenerative processes are significantly less pronounced in more constrained genes and genes that are likely exposed to haploid-phase selection. This may explain why plants retain hundreds of expressed Y-linked genes despite millions of years of Y chromosome degeneration, whereas animal Y chromosomes are almost completely degenerate.

X linkage of AP3A, a homolog of the Y-linked MADS-box gene AP3Y in Silene latifolia and S. dioica

BACKGROUND

The duplication of autosomal genes onto the Y chromosome may be an important element in the evolution of sexual dimorphism. A previous cytological study reported on a putative example of such a duplication event in a dioecious tribe of Silene (Caryophyllaceae): it was inferred that the Y-linked MADS-box gene AP3Y originated from a duplication of the reportedly autosomal ortholog AP3A. However, a recent study, also using cytological methods, indicated that AP3A is X-linked in Silene latifolia.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we hybridized S. latifolia and S. dioica to investigate whether the pattern of X linkage is consistent among distinct populations, occurs in both species, and is robust to genetic methods. We found inheritance patterns indicative of X linkage of AP3A in widely distributed populations of both species.

CONCLUSIONS/SIGNIFICANCE

X linkage of AP3A and Y linkage of AP3Y in both species indicates that the genes' ancestral progenitor resided on the autosomes that gave rise to the sex chromosomes and that neither gene has moved between chromosomes since species divergence. Consequently, our results do not support the contention that inter-chromosomal gene transfer occurred in the evolution of SlAP3Y from SlAP3A.

Evolutionary strata in a small mating-type-specific region of the smut fungus Microbotryum violaceum

DNA sequence analysis and genetic mapping of loci from mating-type-specific chromosomes of the smut fungus Microbotryum violaceum demonstrated that the nonrecombining mating-type-specific region in this species comprises approximately 25% ( approximately 1 Mb) of the chromosome length. Divergence between homologous mating-type-linked genes in this region varies between 0 and 8.6%, resembling the evolutionary strata of vertebrate and plant sex chromosomes.

Silene as a model system in ecology and evolution

The genus Silene, studied by Darwin, Mendel and other early scientists, is re-emerging as a system for studying interrelated questions in ecology, evolution and developmental biology. These questions include sex chromosome evolution, epigenetic control of sex expression, genomic conflict and speciation. Its well-studied interactions with the pathogen Microbotryum has made Silene a model for the evolution and dynamics of disease in natural systems, and its interactions with herbivores have increased our understanding of multi-trophic ecological processes and the evolution of invasiveness. Molecular tools are now providing new approaches to many of these classical yet unresolved problems, and new progress is being made through combining phylogenetic, genomic and molecular evolutionary studies with ecological and phenotypic data.

Development of SCAR markers for sex determination in the dioecious shrub Aucuba japonica (Cornaceae)

Two sex-linked fragments were identified by RAPD analyses in the dioecious diploid shrub Aucuba japonica var. ovoidea and were converted into markers of male-specific sequence characterized amplified region (SCAR) markers. PCRs using the primers designed in this study correctly discriminated 24 flowering males and 24 flowering females at higher annealing temperatures (SCAR markers OPA10-424 at 55 °C and OPN11-1095 at 65 °C), although at relatively low annealing temperatures, the fragments were amplified in both males and females. These SCAR primers were also tested to see whether they were applicable to sex identification in the conspecific tetraploid Aucuba japonica var. japonica. One set pf SCAR primers could be used for sex identification even in this tetraploid variety, although the other failed. The SCAR markers developed in this study will provide a powerful tool in identifying the sex of immature plants of dioecious A. japonica, which is a commercially valuable shrub due to its conspicuous fruits.

The Y chromosome-specific STS marker MS2 and its peripheral regions on the Y chromosome of the dioecious plant Silene latifolia

Sex determination in Silene latifolia uses the XX/XY system. The recent evolution of dioecy in S. latifolia provides a unique opportunity to study the early stages of Y chromosome evolution. However, the current Y chromosome map still contains many large gaps with no available markers. In this study, a sequence tagged site (STS) marker, MS2, was isolated and mapped to the same locus as L8 on the Y chromosome. To investigate the peripheral regions of MS2, a bacterial artificial chromosome (BAC) library was constructed from a male plant, and the BAC clone containing MS2 (MS2-9d12F) was isolated from 32 640 clones with an average insert size of 115 kb. A 109-kb insert of the BAC clone was analyzed. BLASTX analysis showed 11 sequences similar to some known proteins, most of which are retrotransposon-like elements. The ORF Finder predicted 9 ORFs within MS2-9d12F. RT-PCR analyses revealed that only 4 of the 9 predicted ORFs are expressed in both male and female plants. These 4 ORFs are candidates for genes having counterparts on both the X and Y chromosomes. Dot-matrix plot analysis and a BLASTN search revealed LTR-like sequences close to the retrotransposon-like elements and high similarity to 3 known genomic sequences of S. latifolia. These results suggest an accumulation of retrotransposons and segmental duplications in peripheral regions of MS2 during the early stage of sex chromosome evolution.

A selective sweep in the chloroplast DNA of dioecious silene (section Elisanthe)

Gene flow occurs predominantly via pollen in angiosperms, leading to stronger population subdivision for maternally inherited markers, relative to paternally or biparentally inherited genes. In contrast to this trend, population subdivision within Silene latifolia and S. dioica, as well as subdivision between the two species, is substantially lower in maternally inherited chloroplast genes compared to paternally inherited Y-linked genes. A significant frequency spectrum bias toward rare polymorphisms and a significant loss of polymorphism in chloroplast genes compared to Y-linked and autosomal genes suggest that intra- and inter-specific subdivision in the chloroplast DNA may have been eroded by a selective sweep that has crossed the S. latifolia and S. dioica species boundary.

Sex chromosome-linked genes in plants

Recent studies of plant sex chromosome-linked genes have revealed many interesting characteristics, although there are limited reports about heteromorphic sex chromosomes in flowering plants. Sex chromosome-linked genes in angiosperms have been characterized mainly in the dioecious plant Silene latifolia. Although all such genes were isolated from transcripts of male flower buds of S. latifolia, most seem to be housekeeping genes except for the petal- and stamen-specific MADS box gene on the Y chromosome (SlAP3Y) and the male reproductive organ-specific gene on the X chromosome (MROS3X). Recent evolutionary studies have revealed at least three evolutionary strata on the X chromosome that are related to stepwise loss of recombination between the sex chromosomes. Moreover, genetic maps showed conservation of gene organization on the X chromosome in the genus Silene and substantial pericentric inversion between the X and Y chromosomes of S. latifolia during evolution. A comparison between paralogs on the sex chromosomes revealed that introns of the Y-linked genes are longer than those of X-linked paralogs. Although analyses of sex chromosome-linked genes suggest that degeneration of the Y chromosome has occurred, the Y chromosome in flowering plants remains the largest in the male genome, unlike that of mammals. Accumulation of repetitive sequences and the entire chloroplast genome on the Y chromosome appear to have contributed to this large size. However, more detailed studies will be required to help explain the basis for the fact that heteromorphic sex chromosomes in angiosperms are large.

Evolutionary strata on the X chromosomes of the dioecious plant Silene latifolia: evidence from new sex-linked genes

Despite its recent evolutionary origin, the sex chromosome system of the plant Silene latifolia shows signs of progressive suppression of recombination having created evolutionary strata of different X-Y divergence on sex chromosomes. However, even after 8 years of effort, this result is based on analyses of five sex-linked gene sequences, and the maximum divergence (and thus the age of this plant's sex chromosome system) has remained uncertain. More genes are therefore needed. Here, by segregation analysis of intron size variants (ISVS) and single nucleotide polymorphisms (SNPs), we identify three new Y-linked genes, one being duplicated on the Y chromosome, and test for evolutionary strata. All the new genes have homologs on the X and Y chromosomes. Synonymous divergence estimated between the X and Y homolog pairs is within the range of those already reported. Genetic mapping of the new X-linked loci shows that the map is the same in all three families that have been studied so far and that X-Y divergence increases with genetic distance from the pseudoautosomal region. We can now conclude that the divergence value is saturated, confirming the cessation of X-Y recombination in the evolution of the sex chromosomes at approximately 10-20 MYA.