Eichhornia Paniculata Research Articles

Variation in Outcrossing Rates in Eichhornia paniculata: Temporal Changes in Populations of Contrasting Style Morph Structure

Abstract Surveys of mating‐system parameters in populations of the annual, self‐compatible, tristylous, emergent aquatic, Eichhornia paniculata (Pontederiaceae) from N.E. Brazil and Jamaica have indicated that the species exhibits a wide range of outcrossing rates. To investigate whether temporal variation in outcrossing rate was also a feature of populations, open‐pollinated families were sampled from five populations of contrasting style morph structure from N.E. Brazil over three consecutive years (1987–1989). Multilocus estimates of outcrossing rate (t) were obtained from assays of isozyme polymorphisms using starch gel electrophoresis. There was significant variation both among populations and between years in the frequency of outcrossing. Outcrossing in three tristylous populations was high (t > 0.80), with relatively small fluctuations occurring over the three‐year sampling period. In contrast, in a dimorphic and monomorphic population considerable self‐fertilization occurred and the frequency of outcrossing declined significantly from 1987 to 1989 in both populations. In the dimorphic population, increased selfing was associated with a marked reduction in population size and an increase in the frequency of selfing variants of the mid‐styled morph. The significance of temporal variation in outcrossing frequency in plant populations is discussed in relation to its effect on population genetic structure and recent models of mating‐system evolution.

Multiple origins of self‐fertilization in tristylous Eichhornia paniculata (Pontederiaceae): Inferences from style morph and isozyme variation

AbstractPopulations of Eichhornia paniculata (Pontederiaceae) exhibit a wide range of mating systems, from predominant outcrossing to high levels of self‐fertilization. The origin of self‐fertilization in this tristylous species is associated with the loss of style‐length morphs from populations and the spread of self‐pollinating, floral variants. We examined geographic variation in style morph and allozyme frequencies to determine whether the loss of style morphs and transition to selfing could have multiple origins in E. paniculata. Surveys of floral variation in 167 populations from six states in northeastern Brazil revealed that at least one style morph was absent from 29.3%. Non‐trimorphic populations occurred in all states and ranged in frequency from 9% in Ceará to 68% in Alagoas. Selfing variants occurred in 8.5% and 55% of trimorphic and non‐trimorphic populations, respectively, and were distributed among five of six states with primary concentrations in Alagoas and Pernambuco. A comparison of electrophoretic variation at 24 isozyme loci in 28 trimorphic, 13 dimorphic and 3 monomorphic populations indicated that non‐trimorphic populations contained 84% of the allelic variation present in trimorphic populations and were markedly differentiated from one another. Analyses of genetic distance and the distribution of rare alleles indicated that non‐trimorphic populations were often more similar to neighbouring trimorphic populations than to one another. Populations with selfing variants occurred at low frequency in three genetically distinct parts of the range. These results, in combination with genetic and morphological evidence suggest that style morphs are lost repeatedly from populations of E. paniculata and that selfing variants may have originated on at least three separate occasions in northeastern Brazil.

Pollinator visitation in populations of tristylous Eichhornia paniculata in northeastern Brazil.

The frequencies of floral morphs in populations of tristylous Eichhornia paniculata often deviate from the theoretical expectation of equality. This variation is associated with the breakdown of tristyly and the evolution of self-fertilization. Differences in morph frequencies could result from selection pressures due to variable levels of insect visitation to populations and contrasting foraging behavior among the floral morphs. We estimated pollinator densities in 16 populations and quantified visitation sequences to morphs in five populations of E. paniculata in northeastern Brazil. Foraging behavior among floral morphs was measured as the frequency of visits to morphs relative to their frequency in the population (preference) and number of flights between inflorescences of the same versus different morphs (constancy). Pollinator density (number/m2/minute) was not correlated with population size, plant density or morph diversity. Pollinator densities varied most among populations of less than 200 plants. Whether pollinators discriminated among the morphs, depended on whether they primarily collected nectar or pollen. In four populations, nectar-feeding bees (Ancyloscelis and Florilegus spp.) and butterflies showed no consistent preference or constancy among the morphs. In contrast, pollen-collecting bees (Trigona sp.) visited a lower proportion of longstyled inflorescences than expected and tended to visit more mid-and short-styled inflorescences in succession, once they were encountered. Pollinator constancy for morphs did not result from differences in inflorescence production or spatial patchiness among the morphs. Although non-random pollinator visitation to morphs in heterostylous populations could potentially affect mating and hence morph frequencies, the observed visitation patterns in this study do not provide evidence that pollinators play a major role in influencing floral morph frequencies.

EXPERIMENTAL STUDIES ON THE FUNCTIONAL SIGNIFICANCE OF HETEROSTYLY.

Heterostyly has been viewed as both an antiselfing device and a mechanism that increases the proficiency of pollen transfer between plants. We used experimental manipulation of the morph structure of garden populations of self-compatible, tristylous Eichhornia paniculata to investigate the function of floral polymorphism. Outcrossing rates (t), levels of intermorph mating (d), and morph-specific male and female reproductive success were compared in replicate trimorphic and monomorphic populations. In trimorphic populations, t and d averaged 0.81 (2 SE = 0.03) and 0.77 (2 SE = 0.03) respectively, with no difference in either parameter among morphs. Ninety-five percent of outcrossed seeds were therefore the result of intermorph fertilizations. Male reproductive success of the long-styled morph was low, especially in comparison with plants of the short-styled morph. Outcrossing rates for each morph were higher in trimorphic than monomorphic populations where t averaged 0.71 (2 SE = 0.01), 0.30 (2 SE = 0.04) and 0.43 (2 SE = 0.1) for the long-, mid-, and short-styled morphs, respectively. Seed set was lower in monomorphic populations, particularly those composed of the L morph, reflecting reduced pollen deposition. Floral polymorphism therefore increased both outcrossing rate and fecundity but the magnitude of the differences varied among morphs. If the ancestral condition in heterostylous groups resembled the L morph, as has been suggested, data from this study suggests that the selective basis for the establishment of floral polymorphism could have been increased pollen transfer rather than higher levels of outcrossing.

Floral variation in Eichhornia paniculata (Spreng.) Solms (Pontederiaceae) II. Effects of development and environment on the formation of selfing flowers

AbstractGenotypes of the mid‐styled morph of tristylous Eichhornia paniculata (Spreng.) Solms (Pontederiaceae) exhibit developmental instability in the position of short‐level stamens under both field and glasshouse conditions. Elongation of one of the stamens to a position adjacent to the stigma results in automatic self‐pollination of flowers. This modification initiates subsequent changes to floral morphology leading to the evolution of predominant self‐fertilization in E. paniculata. The influence of genetic, developmental and environmental factors on the expression of stamen instability was investigated in experiments with genotypes from two populations from northeast Brazil and interpopulation hybrids. Genotypes from the three sources differed significantly in the degree and pattern of stamen instability expressed under uniform growing conditions. Significant position effects in the production of modified flowers were detected among genotypes using logistic regression techniques. Modified flowers were most frequently produced on later inflorescence branches in the flowering sequence and at proximal flower positions within an inflorescence branch. However, these patterns were complex, varying among genotypes and experimental conditions. Stamen modification increased in clones grown under water stress or at high temperature, demonstrating a significant environmental component to floral instability.

Floral Manipulations Reveal the Cause of Male Fitness Variation in Experimental Populations of Eichhornia paniculata (Pontederiaceae)

In experimental populations of self-compatible, tristylous Eichhornia paniculata plants of the long-styled morph (L) consistently sire fewer seeds on the mid-styled morph (M) than do plants of the short-styled morph (S). This mating asymmetry results in gender differences between the L and S morphs. Observations of pollinators and floral manipulations of genetically marked plants in experimental populations were used to evaluate three potential causes of differential siring success: pollinators prefer the S morph leading to higher male success; long styles interfere with pollen pick-up by insect visitors; and pollen from long-level anthers of the S morph sires more seeds on the M morph than pollen from short-level anthers of the L morph (...)

Variation in Outcrossing Rates in Eichhornia paniculata: The Role of Demographic and Reproductive Factors*

Abstract A survey of 32 populations of the self‐compatible, tristylous, aquatic, Eichhornia paniculata (Pontederiaceae), from N. E. Brazil and Jamaica, was undertaken to examine the role of demographic and reproductive factors on variation in outcrossing rate (t), and to investigate the association between t and population genetic structure. Multi‐locus outcrossing rates, estimated using isozyme techniques, varied widely (t̂= 0.002–0.960) among populations and were uniformly distributed across the entire range of t. Sixty percent of variation in t̂ was explained by style morph diversity (E) and the frequency of selfing variants within populations (M). Population size (N) and plant density (d) also accounted for a significant portion of variation in t̂, particularly in Jamaica, where variation in style morph diversity was low.Outcrossing rates were significantly correlated with the proportion of loci that were polymorphic (P), the average number of alleles per polymorphic loci (Na), the average observed heterozygosity, (Ho), and the inbreeding coefficient (f̂). A strong regional effect was evident in the association of f̂ with P and Na. This largely results from the confounding effect of genetic bottleneck(s) and high levels of self‐fertilization associated with the colonization of Jamaica. Comparisons of f̂ with the equilibrium inbreeding coefficient, (Feq), indicated that outcrossing populations had a significant deficiency of heterozygotes, while selfing populations had an excess. This demonstrates that levels of heterozygosity cannot be accounted for by the mating system alone. Outcrossing rate variation in E. paniculata is controlled by a complex interplay of local demographic and genetic factors. These influences operate within contrasting regional contexts as a result of the different evolutionary histories of populations in N. E. Brazil and Jamaica.

Floral variation in Eichhornia paniculata (Spreng.) Solms (Pontederiaceae): I. Instability of stamen position in genotypes from northeast Brazil

AbstractFloral instability in the mid‐styled morph of tristylous Eichhornia paniculata (Spreng.) Solms (Pontederiaceae) was investigated in 14 genotypes from three Brazilian populations of contrasting morph structure (trimorphic, dimorphic, and monomorphic) and levels of inbreeding. Floral instability was described using uni‐ and multivariate methods, in terms of the degree and pattern of variation and covariation in 14 floral measurements from plants grown under uniform glasshouse conditions. The greatest variability was exhibited by filaments of the two lateral short‐level stamens, within and between genotypes from the dimorphic and monomorphic populations. Elongation of one of the stamens to a position adjacent to the stigma results in automatic self‐pollination of flowers. Genotypes were found to differ more in the pattern of covariation between measurements than in multivariate variability. Differences between genotypes in the correlation structure of floral traits were most pronounced in the monomorphic population, and primarily involved measurements of unmodified, rather than modified, stamens. There was no consistent relationship between multivariate variability and whether genotypes produced modified flowers. This pattern suggests that short‐stamen modification arises from changes at specific loci rather than through generalised floral instability brought about by genome‐wide homozygosity.

THE DISSOLUTION OF A COMPLEX GENETIC POLYMORPHISM: THE EVOLUTION OF SELF-FERTILIZATION IN TRISTYLOUS EICHHORNIA PANICULATA (PONTEDERIACEAE).

Eichhornia paniculata (Pontederiaceae) displays a wide range of outcrossing levels as a result of the dissolution of the tristylous genetic polymorphism and the evolution of semihomostyly. Population surveys, comparison of fitness components of the style morphs, and computer simulations were used to investigate the breakdown of tristyly and the selective mechanisms responsible for the evolution of self-fertilization. Of 110 populations surveyed in northeast Brazil and Jamaica, 53% were trimorphic, 25% were dimorphic, and 22% were monomorphic for style morph. The short (S) morph was underrepresented in trimorphic populations and absent from nontrimorphic populations. The mid (M) morph predominated in dimorphic populations and was the only morph in monomorphic populations. Stamen modifications promoting selfing, associated with semihomostyle evolution, were largely confined to the M morph. They were rare in trimorphic populations, common in dimorphic populations, and often fixed in monomorphic populations. Stochastic simulations and comparisons of fruit set in natural populations indicate that founder events, population bottlenecks, and lowered fertility of the S morph due to an absence of long-tongued pollinators can each account for loss of the S morph from trimorphic populations. A reduced level of disassortative mating can accentuate the rate at which the S morph is lost by both random and deterministic processes. Nontrimorphic populations occur at the geographical margins of the region surveyed and tend to be smaller and less dense than trimorphic populations. These observations and the higher fruit set of the M morph relative to the L morph in dimorphic populations suggest that reproductive assurance, favoring selfing variants of the M morph under conditions of low pollinator service, has been of primary importance in the origin of most monomorphic populations. Where pollinator service is reliable, however, automatic selection of selfing genes, aided by mating asymmetries between the morphs, can cause the M morph to spread to fixation in dimorphic populations.

Disassortative mating in tristylous Eichhornia paniculata (Pontederiaceae)

Populations of the tristylous, self-compatible diploid Eichhornia paniculata (Spreng.) Solms. (Pontederiaceae) display wide variation in floral traits likely to influence the mating system. Progeny tests of open-pollinated plants collected from two populations in N.E. Brazil with contrasting structures (trimorphic, dimorphic) were used to investigate mating patterns. The two loci that govern the inheritance of style length were used as marker genes. A model that measures the magnitude of disassortative mating was developed using the progeny test data. Since the model does not distinguish between selling and assortative mating, outcrossing estimates are likely to be underestimated. Using the model it was shown that the mating patterns of the two populations were strikingly different. In the tristylous population a high level of disassortative mating was estimated (t = 0·903) with no significant differences among the floral morphs. In the dimorphic population (L, M) the rate of disassortative mating of the long-styled morph was t = 0·657, whereas all matings in the mid-styled morph were either selfed or assortative. Selfing in the mid-styled morph is due to alteration of the position of short-level stamens resulting in automatic self-pollination. The consequence of this mating asymmetry on the dynamics of selection at loci controlling floral trimorphism is discussed.

Floral trimorphism and monomorphism in continental and island populations of Eichhornia paniculata (Spreng.) Solms. (Pontederiaceae)

Eichhornia paniculata (Spreng.) Solms. (Pontederiaceae) is a short-lived perennial or annual of marshes, seasonal pools and ditches of lowland tropical South America, primarily NE Brazil, and the Caribbean islands of Cuba and Jamaica. Comparisons, made under uniform glasshouse conditions, of populations originating from seed collected in the two regions revealed striking differences in their floral biology and breeding systems. The majority of populations sampled in NE Brazil are tristylous. Floral trimorphism is associated with pollen trimorphism and minor differences in anther size and pollen production among the three stamen levels. Unlike the majority of heterostylous plants the floral morphs of E. paniculata are highly self-fertile. Populations sampled on the island of Jamaica are composed exclusively of self-pollinating, semi-homostylous, mid-styled forms. Flowers from these populations are smaller and less showy, with reduced pollen heteromorphism and significantly fewer pollen grains and ovules per flower, in comparison with trimorphic populations from Brazil. Individual genotypes from Jamaican populations display considerable developmental instability in floral expression, particularly with respect to filament elongation of the lower stamen level. It is proposed that self-pollinating populations of E. paniculata are evolutionarily derived from tristylous ancestors and that the shift in breeding system is favoured at low density, following population bottlenecks, where pollinator service is unreliable.

THE DEVELOPMENTAL BASIS OF TRISTYLY IN EICHHORNIA PANICULATA (PONTEDERIACEAE)

Eichhornia paniculata is a tristylous, self‐compatible, emergent aquatic. A given plant produces flowers with either long, mid or short styles and two levels of stamens equal in length to the styles not found in that flower. Flowers of each morph have two whorls of three tepals, six stamens and three fused carpels. The six stamens differentiate into two sets of three stamens each. A relatively short set, having either short‐ or mid‐level stamens, occurs on the upper side of the flower, while a relatively long set, having either mid‐ or long‐level stamens, occurs on the lower side. Stamen level depends on differences among stamens in filament length and position of insertion on the floral tube. Floral parts arise in whorls of three, but the two stamen whorls do not form the two sets of stamens found in each mature flower. Instead, stamens from both whorls make up a given set. Floral differences among morphs are not present at flower origin or floral organ initiation. Morphological differences arise first among stamen sets. The two sets within a flower differ prior to meiosis in the size, number, and timing of comparable developmental events in the sporogenous cells. After these initial differences arise, anther size diverges. In later developmental stages differences in filament and floral tube length, cell size, and cell number, as well as differences in the length, cell size, and cell number of styles, develop among morphs. This sequence of developmental events suggests that the genes controlling development in different morphs do not control flower and floral organ initiation but are first morphologically visible in sporogenous cell differentiation.