Michael Nicolas

Evidence for Degeneration of the Y Chromosome in the Dioecious Plant Silene latifolia

The human Y--probably because of its nonrecombining nature--has lost 97% of its genes since X and Y chromosomes started to diverge [1, 2]. There are clear signs of degeneration in the Drosophila miranda neoY chromosome (an autosome fused to the Y chromosome), with neoY genes showing faster protein evolution [3-6], accumulation of unpreferred codons [6], more insertions of transposable elements [5, 7], and lower levels of expression [8] than neoX genes. In the many other taxa with sex chromosomes, Y degeneration has hardly been studied. In plants, many genes are expressed in pollen [9], and strong pollen selection may oppose the degeneration of plant Y chromosomes [10]. Silene latifolia is a dioecious plant with young heteromorphic sex chromosomes [11, 12]. Here we test whether the S. latifolia Y chromosome is undergoing genetic degeneration by analyzing seven sex-linked genes. S. latifolia Y-linked genes tend to evolve faster at the protein level than their X-linked homologs, and they have lower expression levels. Several Y gene introns have increased in length, with evidence for transposable-element accumulation. We detect signs of degeneration in most of the Y-linked gene sequences analyzed, similar to those of animal Y-linked and neo-Y chromosome genes.

Independent Origin of Sex Chromosomes in Two Species of the Genus Silene

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.

Premature arrest of the male flower meristem precedes sexual dimorphism in the dioecious plant Silene latifolia

Most dioecious plant species are believed to derive from hermaphrodite ancestors. The regulatory pathways that have been modified during evolution of the hermaphrodite ancestors and led to the emergence of dioecious species still remain unknown. Silene latifolia is a dioecious plant species harboring XY sex chromosomes. To identify the molecular mechanisms involved in female organ suppression in male flowers of S. latifolia, we looked for genes potentially involved in the establishment of floral organ and whorl boundaries. We identified homologs of Arabidopsis thaliana SHOOTMERISTEMLESS (STM) and CUP SHAPED COTYLEDON (CUC) 1 and CUC2 genes in S. latifolia. Our phylogenetic analyses suggest that we identified true orthologs for both types of genes. Detailed expression analyses showed a conserved expression pattern for these genes between S. latifolia and A. thaliana, suggesting a conserved function of the corresponding proteins. Comparative in situ hybridization experiments between male, female, and hermaphrodite individuals reveal that these genes show a male-specific pattern of expression before any morphological difference become apparent. Our results make SlSTM and SlCUC strong candidates for being involved in sex determination in S. latifolia.

The inter-specific hybrid Silene latifolia x S. viscosa reveals early events of sex chromosome evolution.

Summary The dioecious plant species Silene latifolia has a sex determination mechanism based on an active Y chromosome. Here, we used inter‐specific hybrids in the genus Silene to study the effects of gene complexes on the Y chromosome. If the function of Y‐linked genes has been maintained in the same state as in the hermaphrodite progenitor species, it should be possible to substitute such genes by genes coming from a related hermaphrodite species. In the inter‐specific hybrid, S. latifolia×S. viscosa, anthers indeed develop far beyond the early bilobal stage characteristic of XX S. latifolia female plants. The S. viscosa genome can thus replace the key sex determination gene whose absence abolishes early stamen development in females (loss of the stamen‐promoting function, SPF), so that hybrid plants are morphologically hermaphrodite. However, the hybrids have two anther development defects, loss of adhesion of the tapetum to the endothecium, and precocious endothecium maturation. Both these defects were also found in independent Y‐chromosome deletion mutants of S. latifolia. The data support the hypothesis that the evolution of complete gender dimorphism from hermaphroditism involved a major largely recessive male‐sterility factor that created females, and the appearance of new, dominant genes on the Y chromosome, including both the well‐documented gynoecium‐suppressing factor, and two other Y specific genes promoting anther development.

The Effect of Standard and High-Fluence Corneal Cross-Linking (CXL) on Cornea and Limbus

PURPOSE When treating peripheral ectatic disease-like pellucid marginal degeneration (PMD), corneal cross-linking with UV-A and riboflavin (CXL) must be applied eccentrically to the periphery of the lower cornea, partly irradiating the corneal limbus. Here, we investigated the effect of standard and double-standard fluence corneal cross-linking with riboflavin and UV-A (CXL) on cornea and corneal limbus in the rabbit eye in vivo. METHODS Epithelium-off CXL was performed in male New Zealand White rabbits with two irradiation diameters (7 mm central cornea, 13 mm cornea and limbus), using standard fluence (5.4 J/cm(2)) and double-standard fluence (10.8 J/cm(2)) settings. Controls were subjected to epithelial removal and riboflavin instillation, but were not irradiated with UV-A. Following CXL, animals were examined daily until complete closure of the epithelium, and at 7, 14, 21, and 28 days. Animals were killed and a corneoscleral button was excised and processed for light microscopy and immunohistochemistry. RESULTS For both irradiation diameters and fluences tested, no signs of endothelial damage or limbal vessel thrombosis were observed, and time to re-epithelialization was similar to untreated controls. Histological and immunohistochemical analysis revealed no differences in the p63 putative stem cell marker expression pattern. CONCLUSIONS Even when using fluence twice as high as the one used in current clinical CXL settings, circumferential UV-A irradiation of the corneal limbus does not alter the regenerative capacity of the limbal epithelial cells, and the expression pattern of the putative stem cell marker p63 remains unchanged. This suggests that eccentric CXL may be performed safely in PMD.

Majo et al. reply

Replying to: T.-T. Sun, S. C. Tseng & R. M. Lavker 463, 10.1038/nature08805 (2010)Our claim is not that there are no stem cells in the limbus, but that there is more to corneal renewal than the limbus and that the double-dome-shaped structure of the cornea and physical constraints have a crucial impact on cell dynamics.

[Darwin or Lamarck? Understanding the ocular surface and its normal or abnormal differentiation in order to cure ocular surface destruction with corneal opacification].

According to the World Health Organization, 5.1% of blindnesses or visual impairments are related to corneal opacification. Cornea is a transparent tissue placed in front of the color of the eye. Its transparency is mandatory for vision. The ocular surface is a functional unit including the cornea and all the elements involved in maintaining its transparency i.e., the eyelids, the conjunctiva, the lymphoid tissue of the conjunctiva, the limbus, the lacrymal glands and the tear film. The destruction of the ocular surface is a disease caused by : traumatisms, infections, chronic inflammations, cancers, toxics, unknown causes or congenital abnormalities. The treatment of the ocular surface destruction requires a global strategy including all the elements that are involved in its physiology. The microenvironnement of the ocular surface must first be restored, i.e., the lids, the conjunctiva, the limbus and the structures that secrete the different layers of the tear film. In a second step, the transparency of the cornea can be reconstructed. A corneal graft performed in a healthy ocular surface microenvironnement will have a better survival rate. To achieve these goals, a thorough understanding of the renewal of the epitheliums and the role of the epithelial stem cells are mandatory.