EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. III. MATERNAL AND PATERNAL EFFECTS DURING SEEDLING ESTABLISHMENT

PARENTAL EFFECTS ON PROGENY PHENOTYPE IN PLANTS: DISTINGUISHING GENETIC AND ENVIRONMENTAL CAUSES

The experimental measurement of additive genetic variation in plant populations is complicated by the potential for non-Mendelian inheritance. Maternal influences on progeny phenotype resulting from the cytoplasmic inheritance of plastids or RNA transcripts and effects of the maternal environment have consequently been the focus of much research. To exclude or to control for these sources of variation, breeding designs (e.g., cross-factored, nested, or diallel) in which genetically unrelated pollen donors are mated to maternal genotypes have been adopted. Using these designs, some empirical studies have detected statistically significant differences among pollen donors in the mean performance of their pollen (the mature male gametophytes) or in the mean phenotype of their progeny. These statistical effects of pollen-donor identity on pollen performance or progeny phenotype have frequently been interpreted as evidence for additive genetic variance among pollen donors, although patrilineal cytoplasmic inheritance or effects of the paternal environment on pollen performance or gene expression are recognized as alternative explanations. We note that environment-specific selection among developing gametophytes-in which the environment experienced by developing pollen grains (or ovules) provides a selective force causing the differential survival of gametophyte genotypes (analagous to meiotic drive)-is an additional process that may cause genetically based paternal (or maternal) effects on gametophyte performance. If genes selected during this process are expressed in the sporophyte (postfertilization), this process could also influence the phenotype of the diploid progeny. Here, we review the potential causes of statistically significant differences in mean phenotype among the gametophytes or progeny of maternal (seed-bearing) or paternal (pollen-donating) parental plants. We suggest an experimental approach that permits the detection or elimination of selection among developing gametophytes as one such cause. Specifically, the replication of homozygous parental genotypes within and across environments allows the detection and measurement of paternal and maternal environmentally induced effects on gametophyte or offspring phenotype, while eliminating meiotic drive as a source of the phenotypic variation.

THE MAINTENANCE OF SEX BY GROUP SELECTION

The traditional group-selection model for the maintenance of sex is based upon the assumption that the long-term evolutionary benefits of sexual reproduction result in asexual lineages having a higher extinction rate than sexual species. This model is reexamined, as is a related model that incorporates the possibility that sexual and asexual lines differ in their speciation rates. In these models, the long-term advantage of sex is opposed by a strong short-term disadvantage arising from the twofold reproductive cost of producing males. It is shown that once some sexual lines become established, then group selection can act to maintain sex despite its short-term disadvantage. The short-term disadvantage is included in the model by assuming that, if asexual individuals arise by mutation within a previously completely sexual species, then the asexuals quickly displace their sexual conspecifics and the species is transformed to asexuality. The probability of this event is given by the transition rate, u_s . If the value of u_s varies between lineages, then one of the effects of group selection is to favor groups (i.e., species) with the lowest values of u_s . This occurs because lines that do convert to asexuality (because of a high u_s ) are doomed to a high rate of extinction, and in the long term only those that do not convert to asexuality (because of a low u_s ) survive. The net result of group selection is that sex is maintained because of its lower extinction rate (or higher speciation rate) and because asexual mutants only rarely arise.

FITNESS CONSEQUENCES OF MULTIPLE PATERNITY IN WILD RADISH, RAPHANUS SATIVUS

In natural populations, wild radish plants typically mate with 6-8 pollen donors, and seeds of individual fruits are usually sired by 1-4 fathers. Since radish fruits are indehiscent and gravity-dispersed, progeny are most likely to compete with a mixture of full and half siblings. The fitness consequences of single and multiple paternity were investigated in a greenhouse experiment. Seeds of every possible cross in a 5 times 5 reciprocal diallel mating design were assigned to one of three competition regimes (four full siblings, four maternal half siblings, or four unrelated individuals per pot) or were grown as singletons. After 14 weeks, the aboveground biomass of all plants was harvested and oven-dried. The dry weight of singletons was more than three times that of progeny grown in competition, indicating that intraspecific competition had occurred. Full- and half-sib progenies did not differ in mean dry weight. Thus, there was no evidence that multiple paternity enhances this aspect of maternal fitness. However, the competition regime dramatically affected the coefficient of variation in dry weight of progeny within a pot. Weight hierarchies were much more pronounced in pots of half sibs and unrelated neighbors than in pots of full sibs. Also variance in dry weight attributable to sire was greatest in the half-sib and "unrelated neighbors" competition regimes. These results suggest that weight hierarchies reinforce genetic differences among the offspring.

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. V. A FIELD TEST OF THE SIB-COMPETITION LOTTERY HYPOTHESIS

Sexually and asexually derived tillers of Anthoxanthum odoratum were planted directly in the field to test the hypothesis that competition among groups of sexual and asexual siblings favors the maintenance of sexual reproduction in populations. The results showed a substantial fitness advantage for sexual tillers. However, in contrast with the models, the advantage of sex did not increase with increasing numbers of colonists in the patch, there were multiple survivors among colonists, and an advantage was observed even for singly planted tillers. When a truncation-selection scheme was imposed ex post facto on the data, the relative performance of sexual tillers was similar to that predicted by the Bulmer (1980) model, suggesting that sib-competition models fail due to the violation of the assumption of truncation selection. The advantage of sex was not correlated with the presence of other species, total percentage cover, or species diversity, although sites where sex was favored were physically clustered.

ENVIRONMENTAL AND GENETIC CONSTRAINTS ON ADAPTIVE POPULATION DIFFERENTIATION IN ANTHOXANTHUM ODORATUM

Maximum-likelihood estimates of environmental and broad sense genetic (co)variance components were obtained for the growth and reproductive output of clones of the grass Anthoxanthum odoratum. The clones were transplanted between a mesic and a xeric field site and across-environment genetic correlations were used to estimate the strength of genotype-environment interaction. Significant across-environment clonal covariance matrices were found for several traits, including lifetime reproductive output in one population. None of the matrices differed significantly between populations. Significant within-site clonal variation was found, but there was no significant across-environment clonal covariation. Most broad sense heritability estimates of character states within sites were small (median = 0.12), suggesting that only a slow response to selection is possible. All significant within-site clonal correlations between growth and reproductive output were positive, although the pattern of negative clonal correlations suggests that there may be a cost to first year reproduction, which might constrain future selection response.

THE EFFECT OF DISTANCE FROM THE PARENTAL SITE ON OFFSPRING PERFORMANCE AND INBREEDING DEPRESSION IN IMPATIENS CAPENSIS: A TEST OF THE LOCAL ADAPTATION HYPOTHESIS

If microgeographic variation in selection within a natural plant population has resulted in local adaptation, then offspring fitness should decline with distance from the parental site. If outcrossed progeny are less well-adapted to the parental environment than inbred progeny, but perform better in environments different from that of the parent, then the fitness of inbred progeny relative to outcrossed progeny should decrease with dispersal distance from the parent. To test these predictions, we collected seedlings at 10-m intervals from a 40 times 40-m permanent grid in a natural population of Impatiens capensis, grew them in a greenhouse, and crossed them to produce outcrossed chasmogamous seeds. Seedlings from outcrossed chasmogamous and self-fertilized cleistogamous seeds were planted back into the source population in the original site of their maternal parents and in arcs 3 and 12 m from the parental location and censused weekly for survival and reproduction. The fitness of inbred offspring declined significantly and the magnitude of observed inbreeding depression increased with distance from the parental site, supporting the local adaptation hypothesis.

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. IV. EFFECT OF NEIGHBOR RELATEDNESS AND APHID INFESTATION ON SEEDLING PERFORMANCE

To investigate the effect of neighbor relatedness in seedling populations, propagules of six Anthoxanthum odoratum parents, produced from a reciprocal diallel cross, were planted into the parental source population, a mown field. The propagules were either surrounded by four unrelated neighbors in a 1 cm square, four sibling neighbors, or no neighbors. About 45% of the emerging seedlings were attacked by aphids (Schizaphis graminum). Aphid infestation significantly reduced seedling survivorship and did not differ with parental genotype or neighbor relatedness; plants without neighbors had a slightly higher infestation risk. Plants without neighbors had lower survivorship than those surrounded by unrelated neighbors although this difference was only significant for plants infested by aphids. When infested by aphids, plants surrounded by siblings had lower survivorship than plants surrounded by non-relatives, suggesting the operation of minority advantage. These results are consistent with the frequency-dependent selection hypothesis for the evolution and maintenance of sexual reproduction.

Quantitative genetic study of the adaptive process

The additive genetic variance with respect to absolute fitness, VA(W), divided by mean absolute fitness, , sets the rate of ongoing adaptation. Fisher's key insight yielding this quantitative prediction of adaptive evolution, known as the Fundamental Theorem of Natural Selection, is well appreciated by evolutionists. Nevertheless, extremely scant information about VA(W) is available for natural populations. Consequently, the capacity for fitness increase via natural selection is unknown. Particularly in the current context of rapid environmental change, which is likely to reduce fitness directly and, consequently, the size and persistence of populations, the urgency of advancing understanding of immediate adaptive capacity is extreme. We here explore reasons for the dearth of empirical information about VA(W), despite its theoretical renown and critical evolutionary role. Of these reasons, we suggest that expectations that VA(W) is negligible, in general, together with severe statistical challenges of estimating it, may largely account for the limited empirical emphasis on it. To develop insight into the dynamics of VA(W) in a changing environment, we have conducted individual-based genetically explicit simulations. We show that, as optimizing selection on a trait changes steadily over generations, VA(W) can grow considerably, supporting more rapid adaptation than would the VA(W) of the base population. We call for direct evaluation of VA(W) and in support of prediction of rates adaptive evolution, and we advocate for the use of aster modeling as a rigorous basis for achieving this goal.

Temperature-induced maternal effects and environmental predictability

Maternal effects could influence the persistence of species under environmental change, but the adaptive significance of many empirically estimated maternal effects remains unclear. Inferences about the adaptive significance of maternal effects depend on the correlation between maternal and offspring environments, the relative importance of frequency- or density-dependent selection and whether absolute or relative fitness measures are used. Here, we combine the monitoring of the environment over time with a factorial experiment where we manipulated both the maternal and offspring environment in a marine bryozoan (Bugula neritina). We focused on temperature as our environmental variable as temperature commonly varies over short time scales in nature. We found that offspring from mothers kept in warmer water were smaller and more variable in size, but had increased dispersal potential and higher metamorphic success than offspring from mothers kept in cooler water. Our results suggest that, under frequency- or density-independent selection, mothers that experienced higher temperatures compared with lower temperatures were favoured. Under frequency- or density-dependent selection, there were indications that mothers that experienced higher temperatures would be favoured only if their offspring encountered similar (warmer) temperatures, though these results were not statistically significant. Analysis of time series data on temperature in the field shows that the maternal thermal environment is a good predictor of the temperatures offspring are likely to experience early in life. We suggest that future studies on maternal effects estimate environmental predictability and present both absolute and relative estimates of maternal fitness within each offspring environment.

Why are there so many theories for sex, and what do we do with them?

It is widely known that there exists a multitude of possible explanations for the maintenance of sex; however, it is less clear how to handle such an explanatory pluralism. In this paper, we address one older and one more recent discussion on what might constitute a good theory for sex and find that they reflect a trade-off between maximizing the scientific virtues of generalism, realism, and precision. A historical analysis indicates that varying research interests and research backgrounds of the different biologists shape the trade-off. We use the reflection on the trade-offs in order to understand the existence of the diversity of theories in the field and discuss how to address the explanatory pluralism. We find that the existence of multiple theories for sex, that is, explanatory pluralism, is not surprising or embarrassing but can be seen as a resource. Still, it is important to clarify the possibilities of integration of different theories. Integration between certain theories might be complex, however, as they involve models and theories from different disciplines that have diverged historically in both conceptual and methodological aspects.

Genetic evidence of frequent long-distance recruitment in a vertebrate-dispersed tree

The importance of dispersal for the maintenance of biodiversity, while long-recognized, has remained unresolved. We used molecular markers to measure effective dispersal in a natural population of the vertebrate-dispersed Neotropical tree, Simarouba amara (Simaroubaceae) by comparing the distances between maternal parents and their offspring and comparing gene movement via seed and pollen in the 50 ha plot of the Barro Colorado Island forest, Central Panama. In all cases (parent-pair, mother-offspring, father-offspring, sib-sib) distances between related pairs were significantly greater than distances to nearest possible neighbours within each category. Long-distance seedling establishment was frequent: 74% of assigned seedlings established > 100 m from the maternal parent [mean = 392 +/- 234.6 m (SD), range = 9.3-1000.5 m] and pollen-mediated gene flow was comparable to that of seed [mean = 345.0 +/- 157.7 m (SD), range 57.6-739.7 m]. For S. amara we found approximately a 10-fold difference between distances estimated by inverse modelling and mean seedling recruitment distances (39 m vs. 392 m). Our findings have important implications for future studies in forest demography and regeneration, with most seedlings establishing at distances far exceeding those demonstrated by negative density-dependent effects.

EFFECT OF THE PARENTAL NUTRIENT REGIME ON GROWTH OF THE PROGENY IN ABUTILON THEOPHRASTI (MALVACEAE)

Two genets of Abutilon theophrasti were clonally replicated and grown to maturity in a glasshouse at two levels of nutrient supply. The seeds produced were weighed and the resulting seedlings exposed to two nutrient levels during development. The progeny plants were harvested on three occasions, and leaf areas and dry weights of the different plant parts were determined. At the early stages of growth an increase in the maternal nutrient supply significantly increased seedling height, cotyledon area, and leaf areas, and seed weight had a significant effect on several traits. The maternal nutrient addition had no significant effect at a later stage of growth (35 d after sowing), but at 56 d after sowing it did affect offspring leaf areas and dry weights. Significant interaction terms indicate that the response to parental nutrient addition may depend both on genotype and on the nutrient status of the progeny. Different plant characters are differentially sensitive to maternal conditions and these may be expressed at different stages of development.

Variation in seed and seedling traits in Pithecellobium pedicellare, a tropical rain forest tree

Pithecellobium pedicellare, a mimosoid legume, is a large canopy tree in the tropical rain forests of Costa Rica. We examined the pattern of variation in seed weight, germination date, hypocotyl length (stem), and rachis length (the first leaf) of the seedlings in this species. Seeds collected from widely dispersed individual trees at the La Selva Biological Station, Costa Rica, were randomly planted in blocks, and grown under controlled, indoor conditions for about 2 weeks. There-fore, we were able to quantify the effects of maternal family on mean seed weight and the effects of maternal family and microenvironment on the remaining traits examined. A significant effect of maternal family was detected for all traits. In particular, the maternal effects on germination date and seedling size traits which were consistently significant even after controlling the initial seed weight may indicate that the maternal effects reflect, at least to some extent, maternal genetic control over these traits. Despite overall strong maternal effects, the performance of maternal siblings, such as the rachis length, differed among blocks. The sensitivity of maternal siblings to the local environments may contribute to the maintenance of genetic variability in this highly outcrossing tropical species.

Genetic analysis of ecologically relevant morphological variability in Plantago lanceolata L. : 5. Diallel analysis of two natural populations

Morphological variability was studied in two populations of Plantago lanceolata using diallel analysis. In each population, reciprocal crosses between all possible pairs of ten plants were made. In the greenhouse, six members of each family were grown and many characters were measured. Using the model of Cockerham and Weir, the contributions of the different genetic variance components were calculated. From earlier papers it was postulated in advance to what extent and by which effect the characters in both populations were genetically determined. The populations had been differentiated for life history and morphological characters, and varied also in the relative contribution of genetic components to variability. In the Merrevliet (Me) population, where strong biotic selection was assumed, low levels of additive-genetic variability were present and the relative dominance appeared to be high. The contrasting population, Westduinen (Wd), which is abiotically controlled and shows strong environmental variability, possessed higher levels of additive-genetic variability and lower levels of relative dominance. It is possible that differential natural selection has diminished additive-genetic variability to different extents in both populations: plasticity and environmental heterogeneity prevented the loss of additive-genetic variability in Wd, whereas in the stable population, Me, natural selection had the opportunity of not only changing the means of the characters but also of diminishing additive-genetic variability to a great extent.

The measurement of small-scale environmental heterogeneity using clonal transplants of Anthoxanthum odoratum and Danthonia spicata

To assess the scale of micro-environmental heterogeneity perceived by two co-occurring grass species, Anthoxanthum odoratum and Danthonia spicata, cloned tillers of each species were planted into the natural habitat at a range of spacings (from 2 cm to more than 2 m apart) and measured for survival and fecundity over three years. A. odoratum responded to heterogeneity at a scale of 4-8 cm and at a scale of 2-8 m but not to intermediate scales. D. spicata did not respond significantly to heterogeneity. However one genotype infected with the systemic fungus Atkinsonella hypoxylon showed a large response to heterogeneity at both small and large spatial scales. The results showed that the scale and level of environmental heterogeneity as measured by its fitness impact depends on the species and genotype involved. The results indicate that small scale environmental heterogeneity could play a role in the maintenance of sexual reproduction in A. odoratum.

Paternal and maternal effects on propagule size in Anthoxanthum odoratum

Propagules (caryopsis plus glumes) from a diallel cross among eight genotypes of Anthoxanthum odoratum were weighed to asses the relative contribution of nuclear, reciprocal, and environmental effects to propagule size. Maternal reciprocal effects were large whereas zygotic nuclear effects were small, although consitently present. There were no interactions between maternal reciprocal and nuclear effects. Environmental effects were large, including environmentally induced maternal effects and within individual variance. These results suggest that response to selection for seed size as a result of sib-competition will be weak, and that seed size differences are relatively unimportant for the maintenance of sexual reproduction in natural populations.