SummaryThe dioecious white campion (Silene latifolia) has been chosen as a working model for sexual development. In this species, sexual dimorphism is achieved through two distinct developmental blocks: inhibition of carpel development in male flowers, and early arrest of anther differentiation in female flowers. The combined advantages of the dioecious system and the availability of a sexual mutant lacking both male and female reproductive organs have been exploited in a molecular subtraction approach using male and asexual flower buds. This resulted in the cloning of 22 cDNA clones expressed in stamens at distinct stages of development. Fourteen of these clones corresponded to genes whose expression was detected in pre-meiotic stamens, a stage of development for which very little information is presently available. Furthermore, the absence of similarities with database sequences for ten clones suggests that they represent novel genes. Functional analysis of each clone will enable their positioning within the reproductive organ developmental pathway(s). In parallel, these clones are being exploited as developmental markers of early differentiation within the flower.

Around 200 BC, the Nordic tribes devised rune symbols to represent the forces of nature. Among these symbols were X-GEBA, the rune of love and sexuality, and Y-FEOH, the rune of success. It was believed that by picking the right rune, the wearer could harness the power the rune represented. Hazard or deep intuition? It turns out that in biology, X and Y symbols define sexual fates and reproduction success.Sex determination systems based on heteromorphic X and Y sex chromosomes are particularly interesting to study from both a developmental and an evolutionary perspective. There are many parallels between the sex determination systems, as well as the organization of sex chromosomes, in different species, even between animals and plants.Two main systems of chromosomal sex determination, XY and X:A (autosomal chromosome) ratio, apparently have evolved many times. Mammals, for example, have the XY system, with a dominant (active) Y chromosome containing the key sex determination function(s), whereas Drosophila melanogaster has an X:A system, where the ratio of X:A chromosomes determines sex by an X chromosome counting system, the Y chromosome being largely dispensable (Hodgkin, 1992).Dioecy is a widespread condition in flowering plants, despite their recent evolutionary origin: 6% of the 240,000 angiosperm species are dioecious and 7% of 13,000 genera of angiosperms include dioecious species, suggesting that it has arisen many times during flowering plant evolution (Renner and Ricklefs, 1995). Dioecy is correlated with perennial climbing growth, wind, or water pollination and has a preponderance in tropical flora. Model species with a chromosomal sex determination are white campion (Silene latifolia; XY system), hop (Humulus lupulus; X:A system), and sorrel species (Rumex spp.) which include both XY-like and X:A systems (Figs. 1 and 2).Plant sex determination has been recently and extensively covered (Ainsworth et al., 1998); therefore, we will mainly concentrate on the contribution of this very particular group of plants to the universal question of sex chromosome evolution.

Among the variety of breeding systems developed by flowering plants, those based on heteromorphic sex chromosomes are the most intellectually challenging in evolutionary terms. This is because, among other things, they enable us to compare sex determination processes between plants and animals. White campion (Silene latifolia, also named Lychnis or Melandrium) is dioecious and, much like us, females are homogametic (XX) and males are heterogametic (XY). Sexual dimorphism in white campion is controlled by two independent developmental pathways operating from the Y chromosome at very early developmental stages and within distinct regions of the flower. In addition, all basic steps in the evolution from the bisexual to the dioecious condition with heteromorphic sex chromosomes are known and available to experimentation in the genus Silene. This group of species has been under scrutiny for more than a century. Such an ideal experimental system enables us to tackle, with novel methodological tools, several classical questions in biology. These include the question of how sexual dimorphism evolved and how dimorphic development is controlled, as well as questions of how sex chromosomes evolve in the absence of meiotic recombination or how male-female genetic conflicts are generated. At the turn of the century, the time is now ripe to have a closer look.

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Haemophilia A (HA) is a bleeding disorder caused by mutations within the X-linked F8 gene. A series of 42 unrelated Moldovan patients with HA had their disease-causative mutation determined to provide clinically valuable genotyping information for a historically underserved population and to utilize factor VIII (FVIII) structural information to analyse the effects of haemophilic missense substitutions. DNA samples were analysed to detect intron 22 and intron 1 inversions followed by heteroduplex analysis of PCR-amplified fragments containing all exonic sequences. Missense sites identified by DNA sequencing were visualized in the recently described crystal structures of human FVIII. Of the 26 different point mutations, 12 were novel. Gel electrophoresis identified samples with a second major DNA band that migrated abnormally; these amplified products were sequenced. Thirteen intron 22 inversions and one intron 1 inversion were found. Two patients had large, partial gene deletions and there were six frameshift, two non-sense, two splicing and 16 missense genotypes. Two subjects with an intron 22 inversion and one with a large, partial gene deletion developed an alloimmune inhibitor. Their localization suggests intra- and possibly inter-molecular interactions that are important for the structural integrity and/or procoagulant function of FVIII.

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