Independent origin of sex chromosomes in two species of the genus Silene

Abstract

Here we introduce a new model species, Silene colpophylla, that could facilitate research of sex chromosome evolution and sex-determining systems. This species is related to the well-established dioecious plant model Silene latifolia. Our results show that S. colpophylla is, similarly to S. latifolia, a male heterogametic species, but its sex chromosomes have evolved from a different pair of autosomes than in S. latifolia. The results of our phylogenetic study and mapping of homologs of S. latifolia X-linked genes indicate that the sex determination system in S. colpophylla evolved independently from that in S. latifolia. We assert that this model species pair will make it possible to study two independent patterns of sex chromosome evolution in related species.

Figure 1.—

Tree for orthologs of the gene SlX3. The tree is based on the sequence amplified using primers 11S18 and 11AS13 (Nicolas et al. 2005). This sequence includes both the coding and noncoding part. The tree based on the neighbor-joining method (divergence observed, pairwise gap removal, 10,000 bootstrap replicates) was constructed using Phylo_win software (Galtier et al. 1996). Branch lengths correspond to total sequence divergence under the model assumed (see bar). Values indicated at the nodes are bootstrap values based on 10,000 replicates. Dioecious species are in boldface underlined characters, the hermaphrodite species are marked with an asterisk, and all the other included species are gynodioecious. The tree shows that the distance of S. colpophylla (and its close relative S. otites) from the dioecious Silene species studied so far (S. diclinis, S. dioica, S. latifolia) is relatively large and that there are nondioecious species that are closely related to either S. colpophylla or S. latifolia.

Figure 2.—

Results of the genetic mapping in S. colpophylla. (A) Linkage group containing three AFLP loci and homologs of the genes that are sex linked in S. latifolia compared with the S. latifolia X chromosome map based on the data of Bergero et al. (2007) (pseudoautosomal region is marked as PAR; homologs are connected by dotted lines). (B) Linkage group containing the sex-determining region (sex) and three AFLP loci.

Figure 3.—

(A) Typical mitotic metaphase plate of S. colpophylla. The observed number of chromosomes (2n = 24) suggests that S. colpophylla is not a polyploid species. No heteromorphic sex chromosomes can be distinguished. (B) Typical mitotic metaphase plate of the S. latifolia male (sex chromosomes are marked by “X” and “Y”). Bars, 5 μm.