Patterns of male sterility within and among populations of the distylous shrub Erythroxylum havanense (erythroxylaceae)

Distyly, a reproductive system characterized by the presence of long-styled (thrum). and short-styled (pin) individuals within a population, has been repeatedly used as a model for the study of the evolution of the reproductive systems in plants. Erythroxylum havanense is a distylous species in which most thrum plants fail to develop a fertile androecium, thus behaving as male-sterile or partially male-sterile plants. Short-styled (thrum) individuals have an increased performance as female parents, thereby compensating for their loss of male fitness. Previous studies of populations within close proximity to each other suggest that E. havanense may be involved in a process of gender specialization in which, unlike other heterostylous species, thrum plants are specializing as females and pins (long-styled) as males. In this paper we describe more general patterns of male sterility, one of the first steps in the evolution of gender specialization, among populations of the distylous shrub Erythroxylum havanense. Pollen germination differed among populations (range 0.52 ± 0.03 to 0.06 ± 0.04), and between morphs. Pollen from pin plants was almost two times (1.89) as fertile as that from thrums (0.36 ± 0.03 and 0.19 ± 0.03, pin and thrums respectively). Thrums were significantly more male sterile in four out of five populations. The population where differences between the floral morphs were not apparent showed the lowest levels of pollen fertility. Accordingly, our results indicate that populations of E. havanense show marked differences in pollen fertility and higher male sterility associated with the thrum morph. We hypothesize that differences between morphs could be explained if restorers of male sterility are linked to the distyly haplotype, while differences in genes associated with male sterility could explain the variation among populations. Overall, the prevalence of thrum-biased male sterility across populations suggests that E. havanense is subject to a process of gender specialization.     

Local genetic differentiation among populations of the mass-flowering tropical shrub Erythroxylum havanense (Erythroxylaceae)

This study is aimed to understand the role of life-history traits in determining the genetic structure of populations. We used Erythroxylum havanense, a distylous shrub with synchronous and massive patterns of flowering and fruiting. We suggest that the high concentration of ephemeral resources produced by mass flowering satiates both pollinators and frugivores, restricting gene flow and leading to genetic differentiation among populations. Using random amplified polymorphic DNAs as genetic markers, we estimated genetic diversity and structure statistics to quantify the amount and distribution of genetic variation within and among five populations from the Pacific coast of Mexico. High levels of genetic variation within populations and significant differentiation among populations located very near to each other were found. Furthermore, spatial autocorrelation analysis indicated the presence of significant genetic structure at short spatial distances. We suggest that by influencing the foraging behavior of pollinators and frugivores, mass flowering may produce the observed patterns of genetic structure, while small differences in flowering or fruiting phenology could further reinforce the isolation of nearby populations.     

Male sterility, fitness gain curves and the evolution of gender specialization from distyly in Erythroxylum havanense

The evolution of dioecy from a monomorphic hermaphroditic condition requires two mutations, one producing females and one producing males. Conversely, a single mutation sterilizing one sexual function in one morph of distylous species would result in functional dioecy because such a mutation also affects the complementary function in the other morph. In this study, we tested these ideas with Erythroxylum havanense, a distylous species with morph-biased male sterility. Based on sex allocation theory we evaluated whether the invasion of thrum females is favoured over the maintenance of this morph cosexuals. Completely male sterile thrum plants obtained higher fitness returns than hermaphrodites or partial male sterile individuals of the same morph, thus favouring the invasion of female thrum plants. We concluded that because fruit production of pin individuals depends on the pollen produced by thrum plants, the invasion of thrum females would result on the evolution of functional dioecy.     

Environmentally induced variation in fecundity compensation in the morph-biased male-sterile distylous shrub Erythroxylum havanense (Erythroxylaceae)

The evolution of male-sterile individuals in hermaphroditic species represents the first step in the evolution of sex specialization. For male-sterile individuals to persist they must have some fitness advantage that compensates for their loss of the male function. Female fecundity also depends on environmental factors as those determining the likelihood of pollination and fertilization. Here we assessed the effects of both male sterility and reproductive synchrony (an environmentally affected trait) on the magnitude of female compensation of Erythroxylum havanense, a distylous shrub with morph-biased male sterility. In vitro measurements of pollen germination showed that thrums were more male sterile than pins. The compensatory advantage of thrums changed by a factor of five depending on flowering synchrony. Flowering in synchrony with the population increased fruit production in both morphs. However, because pins that flowered out of synchrony produced almost no fruits, the reproductive compensation of thrums was higher in these circumstances. Because the magnitude of compensation is frequently considered as a key factor in the evolution of sex specialization, the environmentally induced variation in the magnitude of the reproductive compensation of thrum plants may have profound effects on the evolutionary dynamics of the reproductive system of E. havanense.     

Heterostyly in Erythroxylum coca (Erythroxylaceae)

Erythroxylum coca is a distylous species with a strong self-incompatibility system linked with the floral dimorphism. The two sets of stamens in the flowers are usually unequal in length, but between individuals of both morphs there is considerable variation in the relative qengths of the two sets of stamens, which is unrelated to the incompatibility system. Pin flowers produce more pollen grains than thrum flowers, but thrum pollen is larger than pin pollen. Within each morph the two sets of stamens produce pollen grains of slightly different diameter. Erythroxylum novogranatewe is also distylous. Pin flowers of E. novogranatense var. novogranatense are partially self-compatible, while thrum flowers of E. novogranatense var. truxillense are self-incompatible. Reports of tristyly and of four different morphs in species of Erythroxylum are probably misinterpretations, resulting from limited sampling, of the continuous variation in the relative lengths of the two sets of stamens.     

The effect of disease on the evolution of females and the genetic basis of sex in populations with cytoplasmic male sterility

The evolution of separate males and females is an important evolutionary transition that has occurred multiple times in flowering plants. While empirical studies have stressed the potential importance of natural enemies and organismal interactions in the evolution of separate sexes, there has been no treatment of natural enemies in the theoretical literature. We investigated the effects of disease on the evolution of females in gynodioecious populations composed of females and hermaphrodites, where sex is determined by the interaction of cytoplasmic male sterility (CMS) and nuclear restorer genes. When females are significantly more resistant than hermaphrodites, disease drives an increase in the frequency of females and sex determination becomes nuclear, creating the pre-conditions for the evolution of separate males and females. However, when females are only moderately more resistant, disease drives changes in the frequency of CMS and restorer alleles, but has little effect on the frequency of females. We discuss our results in the context of the evolution of mating systems and cyto-nuclear epistasis.     

高等植物胞质雄性不育及育性恢复的分子生物学研究进展

综述了几种植物胞质雄性不育的分子机理研究进展,着重介绍了与细胞质雄性不育相关的线粒体连锁位点的分析及育性恢复的几种假说,并对今后的研究进行了讨论。     

Fruit removal and postdispersal survivorship in the tropical dry forest shrub Erythroxylum havanense: ecological and evolutionary implications

We studied the relationship between the removal rate and the spatiotemporal availability of ripe fruits of the tropical deciduous shrub Erythroxylum havanense in western Mexico. We also evaluated the effects of dispersal on seed survival during the first stages of establishment. Fast and early dispersal should be favored in E. havanense, since propagules have more time to grow and accumulate resources before the beginning of the severe dry season. In general, high rates of fruit removal imply faster and earlier dispersal. Thus, plants producing large crops should benefit from high removal rates, which will increase the probability of successful establishment by their progeny. To characterize both individual and population fruiting patterns, we made daily counts of fruits on 51 plants arranged in six clumps of different sizes. The daily number of fruits removed per plant was higher for plants with larger initial crop sizes and larger numbers of ripe fruits on a given day, but decreased as clump size increased. Additionally, we monitored postdispersal survival and germination in an experiment manipulating seed density, distance from adult plants, and seed predation. Early establishment was independent of density or distance, and vertebrate seed predation was the main agent of seed mortality. Our results indicate that the critical variable with respect to fruit removal is the number of fruits a plant produces, large plants having higher dispersal rates. Large plants are also more likely to have more seeds escaping postdispersal seed predation.     

Parental effects and gender specialization in a tropical heterostylous shrub

Abstract.— Male sterility in hermaphroditic species may represent the first step in the evolution toward dioecy. However, gender specialization will not proceed unless the male-sterile individuals compensate for fitness lost through the male function with an increase in fitness through the female function. In the distylous shrub Erythroxylum havanense , thrum plants are partially male-sterile. Using data collected throughout eight years, we investigated whether thrum individuals have an increased performance as female parents, thereby compensating for their loss of male fitness. We found that thrum plants outperformed pins in the probabilities of seed maturation and germination and long-term growth of the seedlings. In turn, pollen from pin plants achieved greater pollen tube growth rates. Our results suggest that the superior performance of the progeny of thrum maternal plants is a consequence of better seed provisioning via effects of the maternal environment, cytotype or nuclear genes. Overall, our results suggest that E. havanense is evolving toward a dioecious state through a gynodioecious intermediate stage. This evolutionary pathway is characterized by an unusual pattern of gender dimorphism with thrums becoming females and pins becoming males. We propose that this pattern may be better explained by the interaction between male-sterility cytoplasmic genes and the heterostyly supergene.     

萝卜CMS不育系与保持系小孢子发生的细胞学研究

研究了萝卜胞质雄性不育系A2、A4及其相应保持系B2、B4的小孢子发生与花药壁发育的细胞学特征.结果表明,不育系A2的绒毡层细胞在四分体时期出现异常,小液泡增多,至单核期汇合形成大液泡,绒毡层细胞异常膨大;小孢子外壁染色浅,细胞壁受到破坏,最后与绒毡层一同降解.不育系A4在减数分裂期即表现出异常,绒毡层异常肥大;花药发育后期,小孢子外壁亦染色较浅;绒毡层细胞融合形成细胞团块侵入药室挤压小孢子,两者一同降解.     

The effects of cytoplasmic male sterility on cytonuclear disequilibria in hybrid zones

Male sterility is studied in hybrid zones by different measures of cytonuclear disequilibria, D, D₁, D₂, and D₃. Of particular interest are the dynamics of disequilibria as the system evolves to equilibrium. Our first model, the hybrid swarm model, yields equilibrium results identical to those observed in a model with random mating. In our second model of a hybrid zone, predictions of the sign pattern of disequilibrium values can be made based on migration values. A characteristic sign pattern may help to distinguish cytoplasmic male sterility (CMS) from other mechanisms of selection. Our simple CMS model with migration is successfully fit to cytonuclear data on a hybrid population of cottonwoods.     

Names, types, and distribution of neotropical species of Erythroxylum (Erythroxylaceae)

A list is provided of all 358 taxa ofErythroxylum described from the Neotropics through the year 2000, including type specimen citations, probable synonymy, geographic distribution, and designation of 41 new lectotypes and one new neotype. The list includes 187 accepted species, eight accepted varieties, 27 infraspecific names of uncertain status, and 136 synonyms. A searchable specimen database and type image archive may be accessed at www.fieldmuseum.org/research_collections/database.htm. T. Plowman, deceased 7 January 1989.     

Pre-dispersal seed predation reduces the reproductive compensatory advantage of thrum individuals in Erythroxylum havanense (Erythroxylaceae)

Male-sterility mutations in hermaphroditic species represent the first step in the evolution of gender specialization. Male-sterile individuals commonly compensate the loss of the male function by increasing the number or quality of seeds. Because the magnitude of compensation determines the maintenance of females within populations and the evolution of sexual specialization, plant-animal interactions may affect these evolutionary processes if animals are sensitive to such reproductive asymmetries. Here we explore the effect of seed predation on the reproductive compensation of Erythroxylum havanense, a distylous shrub with morph-biased partial male sterility, during two consecutive years. Seed predation reduced the compensatory advantage of thrums in 1987, but not in 1988. Annual differences in the intensity of seed predation seem to be accounted for differences in the onset and synchrony of flowering. Thus, although seed predators may reduce the compensatory advantage of thrums, their impact is modulated by the environmental cues triggering flowering and insect emergence.     

Mitochondrial gene expression and cytoplasmic male sterility in sorghum

Variation in sorghum mitochondrial translation products has enabled fertile (Kafir) cytoplasm to be distinguished from Milo cytoplasmic male sterile cytoplasm and from three alternative sources of cytoplasmic male sterile cytoplasm. Mitochondria from Milo cytoplasm synthesised a 65 000 mol. wt. polypeptide which was not synthesised by those from Kafir cytoplasm. In the cytoplasmic male sterile combination of Kafir nucleus in Milo cytoplasm synthesis of this polypeptide was dramatically increased. Mitochondria from two cytoplasmic male sterile lines (Kafir nucleus in IS1112 cytoplasm and Yellow Feterita nucleus in M35-1 cytoplasm) did not synthesise the 65 000 mol. wt. polypeptide but synthesised additional high molecular weight polypeptides (from 54 000 to 82 000 mol. wt.), the major one being 82 000. Mitochondria from cytoplasm IS1112 were also distinguished by synthesis of an additional 12 000 mol. wt. polypeptide. Mitochondria from the cytoplasmic male sterile line Martin nucleus in 9E cytoplasm synthesised an additional 42 000 mol. wt. polypeptide but did not synthesise a 38 000 mol. wt. polypeptide detected in all other cytoplasms. Immunoprecipitation of mitochondrial translation products with antiserum raised against subunit I of yeast cytochrome oxidase tentatively identified the 38 000 mol. wt. polypeptide as subunit I of sorghum cytochrome oxidase. The 42 000 mol. wt. polypeptide was also immuno-precipitated by this antiserum and thus is probably an altered form of cytochrome oxidase subunit I.Analysis of native mitochondrial DNA by agarose gel electrophoresis revealed the presence of two plasmid-like DNA species of molecular weight 5.3 and 5.7 kb in the cytoplasmic male sterile lines Kafir nucleus in cytoplasm IS1112 and Yellow Feterita nucleus in M35-1 cytoplasm. Thus there is a positive correlation between the synthesis of the 82 000 mol. wt. polypeptide and the presence of the additional DNA species.     

Genetic determination of male sterility in gynodioecious Silene nutans

Gynodioecy, the coexistence of female and hermaphrodite plants within a species, is often under nuclear–cytoplasmic sex determination, involving cytoplasmic male sterility (CMS) genes and nuclear restorers. A good knowledge of CMS and restorer polymorphism is essential for understanding the evolution and maintenance of gynodioecy, but reciprocal crossing studies remain scarce. Although mitochondrial diversity has been studied in a few gynodioecious species, the relationship between mitotype diversity and CMS status is poorly known. From a French sample of Silene nutans, a gynodioecious species whose sex determination remains unknown, we chose the four most divergent mitotypes that we had sampled at the cytochrome b gene and tested by reciprocal crosses whether they carry distinct CMS genes. We show that gynodioecy in S. nutans is under nuclear–cytoplasmic control, with at least two different CMSs and up to four restorers with epistatic interactions. Female occurrence and frequency were highly dependent on the mitotype, suggesting that the level of restoration varies greatly among CMSs. Two of the mitotypes, which have broad geographic distributions, represent different CMSs and are very unequally restored. We discuss the dynamics of gynodioecy at the large-scale meta-population level.     

Ecological and genetic factors contributing to the low frequency of male sterility in Chamaecrista Fasciculata (Fabaceae)

Bumble bee pollinated Chamaecrista fasciculata provides pollen as the sole reward to its pollinators. Male sterility, expressed as an absence or nearly complete absence of pollen production, occurs in low frequency in populations of C. fasciculata. Here we describe experiments, using C. fasciculata, to examine frequently cited determinants of the spread and maintenance of male sterility: compensation and the genetic basis of male sterility. In addition, we examine the role the pollination system plays in determining the reproductive success of the male steriles. Seventeen populations in Maryland, Illinois, and Kansas were surveyed and found to range from 0 to 6% male sterility per population. An artificial population of male-sterile simulants and hermaphrodites was created to examine how the local frequency of nonrewarding male steriles might affect male-sterile female reproductive success. Male steriles performed equally poorly, with respect to seed production, whether surrounded by other male-sterile simulants or hermaphrodites. Compensation was examined by comparison of male steriles and hermaphrodites with respect to several reproductive and nonreproductive characters. Male steriles outperformed hermaphrodites in terms of nonreproductive biomass, but performed equally in terms of ovule number and produced many fewer flowers. The genetic basis of male sterility was examined by performing both intra- and interpopulational crosses of male steriles to hermaphrodites and indicate that male sterility is not purely cytoplasmic. The low frequency of male sterility in C. fasciculata populations may reflect reduced female reproductive success because of pollinator avoidance, lack of reproductive compensation, and a mode of inheritance that is not purely cytoplasmic.     

Cytoplasmic male sterility in barley

The maternal male sterile barley msm1 with or without a dominant gene, Rfmla, which restores male fertility, was studied. Determined with SDS-PAGE, the polypeptide pattern in the anthers of unrestored msm1 plants remains juvenile in the middle of anther development, two major zones being absent or weak. At the stage when anther development stops in msm1 plants, the anther proteins appear to be hydrolyzed to short-chain peptides. Restored plants, heterozygous for the restorer gene, Rfmla, behaved like the near-isogenic normal barley, cv. Adorra. The total leaf protein pattern of young leaf tissue and the chloroplastidic membrane protein pattern are normal in msm1 cytoplasm when studied with this technique. Chlorophyll b is unnecessary for restoration by Rfmla, though the restored plants have a lower chlorophyll a/b ratio than an unrestored plant in the mature stem leaf. Mature stem leaf pieces of unrestored msm1 plants were induced to senesce with 20 mM NaCl solution. This senescence was inhibited by exogenous kinetin. Leaf pieces of restored msm1 plants or those of near-isogenic normal barley behaved in the same way in the NaCl solution as in distilled water. Many features of the physiology of restored plants can be explained as the functions of cytokinins. Kernels of male sterile plants have a more rapid root elongation at germination than near-isogenic normal barley.     

Molecular basis of cytoplasmic male sterility and fertility restoration in rice

Cytoplasmic male sterility (CMS) is a maternally inherited trait that causes dysfunctions in pollen and anther development. CMS is caused by the interaction between nuclear and mitochondrial genomes. A product of a CMS-causing gene encoded by the mitochondrial genome affects mitochondrial function and the regulation of nuclear genes, leading to male sterility. In contrast, the RESTORER OF FERTILITY gene (Rf gene) in the nuclear genome suppresses the expression of the CMS-causing gene and restores male fertility. An alloplasmic CMS line is often bred as a result of nuclear substitution, which causes the removal of functional Rf genes and allows the expression of a CMS-causing gene in mitochondria. The CMS/Rf system is an excellent model for understanding the genetic interactions and cooperative functions of mitochondrial and nuclear genomes in plants, and is also an agronomically important trait for hybrid seed production. In this review article, pollen and anther phenotypes of CMS, CMS-associated mitochondrial genes, Rf genes, and the mechanism that causes pollen abortion and its agronomical application for rice are described.