Sex-determining and floating translocation complexes in Viscutn cruciatum Sieber ex Boiss.(Viscaceae) in southern Spain. Some evolutionary and ecological comments

As in other dioecious species of Viscum, males of V.cruciatum always produce large rings of chromosomes during meiosis due to the existence of permanent translocation heterozygosity. Only rings often (and very rarely 12) chromosomes have been found in this species, probably a relative of V.album in the Mediterranean area. Females form bivalentS. Another two floating translocations–not sex-associated and affecting different chromosomes–have been observed in both male and female plants in some populations. These produce meiotic configurations previously unrecorded in Viscum. This floating heterozygosity seems to be correlated with the aptitude of V.cruciatum to develop on different hosts, especially Prunus. All studied populations showed an excess of female plants, perhaps partly due to facultative agamospermy.     

Understanding plant reproductive diversity

Flowering plants display spectacular floral diversity and a bewildering array of reproductive adaptations that promote mating, particularly outbreeding. A striking feature of this diversity is that related species often differ in pollination and mating systems, and intraspecific variation in sexual traits is not unusual, especially among herbaceous plants. This variation provides opportunities for evolutionary biologists to link micro-evolutionary processes to the macro-evolutionary patterns that are evident within lineages. Here, I provide some personal reflections on recent progress in our understanding of the ecology and evolution of plant reproductive diversity. I begin with a brief historical sketch of the major developments in this field and then focus on three of the most significant evolutionary transitions in the reproductive biology of flowering plants: the pathway from outcrossing to predominant self-fertilization, the origin of separate sexes (females and males) from hermaphroditism and the shift from animal pollination to wind pollination. For each evolutionary transition, I consider what we have discovered and some of the problems that still remain unsolved. I conclude by discussing how new approaches might influence future research in plant reproductive biology.     

雌雄异株物种一把伞南星雌雄株空间分布及性别二态性

雌雄异株是有花植物雌雄性别功能在个体水平上由结合到分离的重要进化转变之一, 发生于被子植物的多个科属。雌雄异株物种的雌雄个体通常在分布、形态等方面存在差异。研究其雌雄个体的空间分布及性别二态性, 是理解雌雄异株物种繁殖策略及其适应与进化的重要前提。天南星属(Arisaema)是天南星科内已报道唯一存在雌雄异株且较为进化的类群, 一把伞南星(A. erubescens)是该属中不同性别个体存在差异的多年生草本植物。本文以湖南省八大公山国家级自然保护区内分布的一把伞南星为研究对象, 通过研究其雌雄个体的空间分布、花期物候、形态结构及访花者类群, 以期明确该物种雌雄植株的空间关联性以及花期物候、形态结构的性别二态性。结果表明: 一把伞南星群体内性比均呈极显著雄性偏倚, 雌雄个体在小尺度范围呈聚集分布, 但在较大尺度上呈随机分布; 雄性个体比雌性个体更早进入花期, 置换多元方差分析显示雌雄植株间形态特征存在显著差异, 其中假茎基径、佛焰苞管部直径、花序基径、花序柄直径等特征对性别二态性有重要贡献; 主要传粉者为白天活动的菌蚊科菌蚊属(Mycetophila)的蕈蚊, 其对雄株的访问频率极显著高于雌株。这些结果表明: 雌雄异株物种一把伞南星不同性别植株的空间关联与其传粉者蕈蚊的活动习性相匹配, 其开花物候、形态特征及传粉者访问等方面的性别二态性是对繁殖成功的保障。     

Inheritance of progeny sex ratio in Urtica dioica

Seed samples collected from female Urtica dioica plants in the field showed considerable inter-family variation in the sex ratio (faction of males). To investigate the inheritance pattern of the sex ratio trait, crosses were performed between individual male and female plants from different sex ratio families. Our results suggest, at least for the families studied here, that maternal parents strongly contribute to the variation in the primary sex ratio. Furthermore, progeny sex ratios from reciprocal crosses were significantly different and resembled the sex ratios produced by their maternal parents. We discuss the possible mechanisms underlying maternal control.     

Effects of inbred/outbred crosses on progeny sex ratio in Silene latifolia (Caryophyllaceae)

Sex ratio polymorphism has been extensively studied in Silene latifolia, but it is neither known whether inbreeding (which is likely to occur under field conditions) affects it, nor which of the proposed mechanisms (Y degeneration, X-linked drive) is more important. Both mechanisms predict reduced pollen performance. In this study, females were crossed with pollen from related and unrelated males in single-donor and two-donor crosses, and the sex ratio of offspring (n = 866, 60 crosses), sons'in vitro pollen germination and sex ratios in parental families were scored. Flowers receiving only unrelated pollen produced a significant excess of sons. Sex ratios were not significantly correlated between generations. Sons'in vitro pollen germination was significantly negatively correlated with the 'sex-ratio phenotype' of maternal grandfathers, but not of fathers. This generation leap may be consistent with X-linked determinants because Y-linked determinants alone cannot explain it (grandfathers, fathers and sons share the same Y chromosome). Further work is required, but inbreeding and limited dispersal may lead to local accumulation of biasing factors, a process potentially countered by conditional shifts to produce more sons in pure outbred crosses.     

The occurrence and structure of apparently bisexual flowers in the date palm, Phoenix dactylifera L. (Arecaceae)

Individuals of Phoenix dactylifera L. have expanded pistillodes or pseudocarpels in staminate flowers. These pseudocarpels are located in the centre of the male flowers and are surrounded by stamens. The gynoecium has the characteristic three carpellate arrangement commonly found in female date palm flowers. Pseudocarpels from male flower buds can expand into parthenocarpic fruit. Histology of the expanded pistillodes or pseudotarpels is similar to that of normal carpels from pistillate plants. These pseudocarpels lack ovules. Nutrient medium containing 10 mg 1^-1 of 2,4-dichlorophenoxyacetic acid or p-chlorophenylacetic acid and 0.3% activated neutralized charcoal enhanced the development and outgrowth of the pseudocarpels of cultured male flowers.     

Sexual differences in reproductive phenology and their consequences for the demography of Baccharis dracunculifolia (Asteraceae), a dioecious tropical shrub

Patterns of phenological variation and reproductive investment were studied in the dioecious shrub Baccharis dracunculifolia DC (Asteraceae), and possible consequences on survivorship were evaluated. The sex ratio was determined in a natural field population (n = 921) of B. dracunculifolia in Belo Horizonte, Brazil. Fifty-two males and 56 females were sampled at random from this population. During the reproductive season of 1999, inflorescence production, shoot growth and mortality, and xylem water potential were recorded for each individual. The population sex ratio was male-biased (1.27 : 1, P < 0.05), and was associated with a higher mortality of female shoots (38.4 vs. 23.1 %, P < 0.05), and individuals (17.8 vs. 11.5 %, P < 0.1), despite lower water stress in female plants. Flowering phenology also differed between the sexes, with males producing more inflorescences, and earlier, than females. Owing to fruit maturation, the number of inflorescences supported by females was higher than that supported by males later in the reproductive season. This occurred during the dry season, and drought stress may have been responsible for the greater female mortality. Thus, the male-biased sex ratio in this population of B. dracunculifolia is probably due to different reproductive functions of males and females. Intersexual differences in reproductive phenology had consequences for plant demography.     

Sex determination and differentiation in Asparagus officinalis L.

The paper summarizes the coordinated researches conducted by three Italian groups in the area of sex determination and differentiation in the dioecious species Asparagus officinalis. Morphological evidence indicates that sex differentiation in Asparagus consists essentially of selective abortion of gynoecium or androecium of initially hermaphroditic floral primordia occurring in genotypically determined male and female individuals. Abortion occurs in pollen-mother cells and anthers in females and in megaspore-mother cells but not in the vegetative tissues of the ovary in males. The differential developmental pathway is accompanied by changes in relative abundance of auxin and cytokinins. The genetic ssytem controlling abortion of male or female organs is apparently monogenic (possibly a bipartite gene) with factor(s) associated with the homomorphic chromosome pair L5. Other genes influence the development of reproductive structures as indicated by the presence of genetic factors controlling stylar growth in male plants. The presence of extensive polymorphism in isoenzyme and DNA restriction fragment length patterns (RFLP) allows the search for markers associated with ‘sex genes’: a locus encoding a malic dehydrogenase (MDH) isoenzyme has been found about 20 cM from sex genes implying that chromosomes in which sex factors are located could pair and recombine. Searches for messages specifically expressed in reproductive structures were conducted by 2D-electrophoresis of existing and newly synthesized polypeptides or of in vitro translation products of poly(A) ⁺RNA from male and female flowers and by isolating specific monoclonal antibodies against sex specific floral antigens.