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Silene latifolia Poiret is a reliable model system for studying many of the classical problems in biology because little difference in its physiological response to environmental stimuli has been found between genders. In this experiment, we studied flowering responses to day length in male and female S. latifolia plants grown at different temperatures. The primary objective was to examine whether flowering in male and female plants was differentially influenced by day length and temperature. We hypothesized that temperature would interact with day length in affecting flowering, because both genders have been found to flower in a wide range of day lengths and temperatures. Plants were grown in environmental chambers at a target temperature of 15/10 or 30/25°C (day/night). Day length was increased by half-hour intervals until all plants had flowered. Results showed that S. latifolia is a qualitative long-day plant with a critical day length of 11 h. Flowering accelerated with increasing day length until 16.5 h, when all plants flowered. Males and females differed in relative flowering at different day lengths, but to a greater extent at high than at low temperature. This study demonstrated that flowering of male and female S. latifolia respond differently to day length at different temperatures. A more uniform experimental protocol, especially in controlled environments, is called for to make this model system a more useful tool in biological studies.

Two SEPALLATA orthologs, SlSEP1 and SlSEP3, were isolated from male flower buds of the dioecious plant Silene latifolia. Both genes are located on autosomes and not on the sex chromosomes. SlSEP1 and SlSEP3 transcripts were detected specifically in male and female flower buds. Quantitative RT-PCR and in situ hybridization revealed that both genes were expressed in young flower meristems, developing petals, male anthers, and female ovules. However, neither transcript was detected in suppressed gynoecia of male flower buds or suppressed anthers of female flower buds. The genes were differentially expressed during anther development, with the SlSEP1 transcript accumulating in anther walls and tapetal cells, and the SlSEP3 transcript accumulating in tetrads as well as anther walls and tapeta. Transcripts of both genes were also detected in flower buds of mutants with deletions of some parts of the Y chromosome, suggesting that neither gene is directly involved in sex determination.

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.

Melandrium album (Silene alba) is a dioecious plant with heteromorphic sex chromosomes (XY system). Sexual dimorphism is a result of developmental blocks in male or female reproductive organ formation within young bipotential flower buds. Progress in understanding the genetic and molecular mechanisms controlling sex determination in this species relies on a detailed description of developmental timing in the two sexes, with emphasis on those early stages during which sexual dimorphism is established. We used a combination of histological and scanning electron microscopy analysis to refine the comparative study and description of the staging of male compared to female flower development. We show that (1) female dimorphism results from modifications in flower meristem organisation, namely a sudden arrest of cell divisions in whorl 4 of male flowers at the time when meristem partitioning is achieved between whorls 3 and 4, and (2) male dimorphism is part of the stamen differentiation process corresponding to stamen arrest at the early sporogenous stage in female flowers. Thus, Melandrium is a natural double ”mutant” that is affected in very early and distinct processes of reproductive organ differentiation. Our results are used to discuss the most likely nature of the specific functions controlling sexual dimorphism in Melandrium.

Silene latifolia is dioecious, yet rare hermaphrodites have been found, and such natural mutants can provide valuable insight into genetic mechanisms. Here, we describe a hermaphrodite-inducing mutation that is almost certainly localized to the gynoecium-suppression region of the Y chromosome in S. latifolia. The mutant Y chromosome was passed through the megaspore, and the presence of two X chromosomes was not necessary for seed development in the parent. This result supports a lack of degeneration of the Y chromosome in S. latifolia, consistent with the relatively recent formation of the sex chromosomes in this species. When crossed to wild-type plants, hermaphrodites performed poorly as females, producing low seed numbers. When hermaphrodites were pollen donors, the sex ratio of offspring they produced through crosses was biased towards females. This suggests that hermaphroditic S. latifolia would fail to thrive and potentially explains the rarity of hermaphrodites in natural populations of S. latifolia. These results indicate that the Y chromosome in Silene latifolia remains very similar to the X, perhaps mostly differing in the primary sex determination regions.

In order to obtain markers for the Y chromosome ofSilene latifolia, we pooled equal weights of leaf tissue from 18 female siblings into one sample and repeated the process with 18 male siblings. Pooling was intended to provide a common genetic background for each sample, leaving the absence or presence of the Y chromosome as the primary difference between the two samples. DNA was extracted from each sample and subjected to polymerase chain reaction (PCR) amplification with arbitrary 10 bp primers. Four of 60 primers used gave an amplification with the male DNA not found among those from the female DNA. Each of these was subsequently shown to provide a reliable marker for the Y chromosome.

Melandrium album (2n = 24), a dioecious species with heteromorphic sex chromosomes (XY, male; XX, female), has a strong genetic commitment for sex determination. So far the in vitro regeneration response in this species has appeared to be restricted to female plants. We report here a procedure for achieving active regeneration from explants and mesophyll protoplasts of selected female plants. Shoots were frequently induced in hormone-free media and in the presence of cytokinins or low concentrations (