One of the most burning topics in Evolutionary Biology concerns the origin and evolution of sex determining systems and sex chromosomes. Traditionally, Drosophila or humans have been study models for this type of analyses. However, sex-determining systems of these organisms date to hundreds of millions of years ago, hampering the access to early stages of the evolutionary pathway, key events to understand the molecular mechanisms involved. On the other hand, some groups of animals (especially fishes), as well as some plant species (Rumex sp., papaya, Silene latifolia), have younger sex-chromosome systems in different evolutionary stages with different coexisting mating systems (hermaphroditism, gynodioecy, dioecy). These features make fishes and plants excellent models for studies on sex determination. In this context, my main research line seek to shed light on different aspects of Reproductive Biology by using as models some plant species such as Rumex sp., papaya or pistachio, and some fish species such as sole, Solea senegalensis, or turbot, Scophthalmus maximus. My experience in both groups of organisms can be summarized as follow: 1) participation in three consecutive national projects on sex determination in Rumex; 2) participation in two international projects on papaya genome sequencing, focusing on sex-determining region sequencing; 3) participation as PI in a project on sex determination in pistachio, recently submitted; and 4) participation in the project INGENIO CONSOLIDER AQUAGENOMICS, for the development of molecular tools to improve fish culture, focusing on sex-determining genes, within the framework of Plan de Incorporación de Doctores of Plan Propio at the University of Granada (Spain).

I´m generally interested in genome evolution in plants, including among others:

Satellite-DNA and sex-chromosomes evolution.-

I have focused part of my research in the analysis of repetitive sequences (mostly satellite DNA) and their evolutionary dinamics. Satellite DNAs are tandemly arrayed, highly repetitive DNA sequences of the eukaryotic genomes located in the constitutive heterochromatin. The repeats comprising a satellite-DNA family do not evolve independently of one another but rather follow concerted evolution. That is, arrays of nonallelic homologous sequences, homogenized by transfer mechanisms such as unequal crossing-over and gene conversion, evolve as a unit, leading to unexpected levels of intraspecific homogenization and interspecific divergence levels. It is particularly interesting to understand mechanisms involved in such as concerted evolution. Especially interesting for that, are those models in which some of these mechanisms are restricted in some sense. Good example of this are sex chromosomes in which recombination is avoided.

Models of evolutionary dynamics for satellite DNA predict its accumulation in chromosomal regions where recombination rates are low and such accumulation has been documented in the nonrecombining Y chromosomes. Sex chromosomes, while having evolved independently in several different groups of organisms, share common evolutionary features. Thus, the gradual suppression of recombination between the sex chromosomes is thought to lead to their progressive divergence and to the erosion of the Y chromosome. The final outcome of this process should be the loss of function of many genes within the Y chromosome and the degeneration of the Y chromosome, principally due to the accumulation of a set of diverse repetitive sequences such as mobile elements and satellite DNAs. However, little is known about how this occurs or about how the absence of recombination affects the subsequent evolutionary fate of the repetitive sequences in the Y chromosome.

We have used as study model, species belonging to genus Rumex (Polygonaceae) that is becoming a very important key in sex-chromosomes and sex-determination evolution studies. This genus includes an exceptional variety of mating and sex-chromosomes systems.


The Exceptional Genus Rumex.-

According to morphological data, genus Rumex is currently divided into four subgenera : Acetosella, Acetosa, Platypodium, and Rumex. Acetosella contains two species, Rumex acetosella (which has several subspecies) and Rumex graminifolius. These species are dioecious and have a sex-determination mechanism based on the presence of an active Y and a simple chromosome system XX/XY. Within the subgenus Acetosa, the section Acetosa is composed of Rumex acetosa and its relatives, which form an homogeneous group of species characterized by similar morphological and karyological characteristics, including an XX/XY1Y2 sexchromosome system plus a sex-determination mechanism based on the X/A balance. However, within the section Americanae of the subgenus Acetosa, there are two species: Rumex paucifolius, which has the XX/XY system, and Rumex hastatulus, which has two chromosomal races, one with the XX/XY (called the ‘‘Texas race’’) and the other with the XX/XY1Y2 (called the ‘‘North Carolina race’’). Also, the second race has an X/A-based sex-determination mechanism, while the XX/XY race has a Y-based one. Furthermore, the subgenus Acetosa contains four additional sections: Scutati, Vesicarii, Hastati, and Afroacetosa. The first two are composed of hermaphroditic and polygamous species. Strikingly, Scutati has a dioecious species, Rumex suffruticosus, for which no chromosomal data were available. We found for this species the presence of a XX/XY system. The sections Hastati and Afroacetosa are composed of polygamous and gynodioecious species as well as a dioecious one, Rumex sagittatus, which lacks differentiated sex chromosomes. Meanwhile, the third subgenus, Platypodium, has one species (and several subspecies), Rumex bucephalophorus, which is hermaphroditic. Finally, the subgenus Rumex is composed of hermaphroditic species, although endemic Hawaiian species such as Rumex giganteus have evolved towards monoecy.


Phylogenetics and Evolution of Dioecy in plants.-

I´m interested in phylogenetic inferences using different molecular markers (both nuclear and chloroplastidial) by integrating bioinformatics tools and morphological or cladistic characters. We revisited the actual sistematics of genus Rumex and gattered important results regarding sex-chromosomes and dioecy evolution. In brief, dioecy in Rumex seems to have evolved once 15-16 mya including gynodioecious intermediates from hermaphroditism. And simple sex-chromosome systems (XX/XY) gave rise to complex ones (XX/XY1Y2) up to two times, one in Rumex acetosa lineage (12-13 mya) and other in American Rumex hastatulus clade.

Gene Duplication and Polyploidy.-

Polyploidy has been demonstrated to be a common feature in plant evolution. In angiosperms it is estimated that this phenomenon ranges from 50 to 80%. Furthermore, among 2 and 4% of speciation events could be explained by genome duplication. It has also been reported that some genetically diploid species are in fact paleopolyploids and recently suggested that many plants, if not all, have polyploid ancestors at some point in their lineage. The remnants of polyploidy events in the current Arabidopsis genome form a large set of duplicated chromosomal segments, which have been identified and ordered in different age classes. Three genome duplication have been dated up to date: among 20-80 mya (namely alpha), 170-235 mya (namely beta) and 300 mya (namely gamma).

These aspects are currently focusing some investigations in other more distant genera as Carica or Brassica within order Brassicales, which contains Arabidopsis species in order to find out which of these duplications are present and then predate their divergence. One of the main questions should also be addressed is how the presence of sex chromosomes affects polyploidy and whole genome duplication events. For that task, Carica papaya is particularly interesting due to the presence of very incipient sex chromosomes.