Evolution of sex chromosomes prior to speciation in the dioecious Phoenix species.

Understanding the driving forces and molecular processes underlying dioecy and sex chromosome evolution, leading from hermaphroditism to the occurrence of male and female individuals, is of considerable interest in fundamental and applied research. The genus Phoenix, belonging to the Arecaceae family, consists uniquely of dioecious species. Phylogenetic data suggest that the genus Phoenix has diverged from a hermaphroditic ancestor which is also shared with its closest relatives. We have investigated the cessation of recombination in the sex-determination region within the genus Phoenix as a whole by extending the analysis of P. dactylifera SSR sex-related loci to eight other species within the genus. Phylogenetic analysis of a date palm sex-linked PdMYB1 gene in these species has revealed that sex-linked alleles have not clustered in a species-dependent way but rather in X and Y-allele clusters. Our data show that sex chromosomes evolved from a common autosomal origin before the diversification of the extant dioecious species.

Chorology of New Caledonian palms and possible evidence of Pleistocene rain forest refugia.

The distribution of the 36 palm species of New Caledonia is studied in relation to several parameters: elevation, rainfall, geological substrate, phytogeographical sectors and vegetation types. The climate (thermal gradient and rainfall) appears to be the principal factor influencing the distribution of palms, the substrate effect being subordinate to the climatic pattern. Nearly all palm populations are included within the 1,500 mm isoline. Maximum levels of species richness and endemism are located in four areas receiving more than 3,000 mm of annual rainfall. We interpret these areas as former Pleistocene refugia of lowland rain forest based on three lines of evidence: 1) all locally endemic lowland palm species and genera are restricted to these areas; 2) local endemics occur on east-facing slopes receiving the highest rainfall and most likely to have sustained rain forests during the driest periods; and 3) several pairs of sister species are disjunct between the southeastern and northeastern high rainfall areas.

Phylogenetic relationships within Araucariaceae based on rbcL gene sequences.

Phylogenetic relationships were determined in the Araucariaceae, which are now found mainly in the Southern Hemisphere. This conifer family was well diversified and widely distributed in both hemispheres during the Mesozoic era. The sequence of 1322 bases of the rbcL gene of cpDNA was determined from 29 species of Araucariaceae, representing almost all the species of the family. Phylogenetic trees determined by the parsimony method indicate that Araucariaceae are well defined by rbcL sequences and also that the monophyly of Agathis or Araucaria is well supported by high bootstrap values. The topology of these trees revealed that Wollemia had derived prior to Agathis and Araucaria. The rbcL phylogeny agrees well with the present recognition of four sections within Araucaria: Araucaria, Bunya, Eutacta, and Intermedia. Morphological characteristics of the number of cotyledons, position of male cone, and cuticular micromorphologies were evaluated as being phylogenetically informative. Section Bunya was found to be derived rather than to be the oldest taxon. Infrageneric relationships of Agathis could not be well elucidated because there are few informative site changes in the rbcL gene, suggesting the more recent differentiation of the species as their fossil records indicate. The New Caledonian Araucaria and Agathis species each formed a monophyletic group with very low differentiation in rbcL sequences among them, indicating rapid adaptive radiation to new edaphic conditions, i.e., ultramafic soils, in the post-Eocene era.