Ranunculus glacialis L.: successful reproduction at the altitudinal limits of higher plant life

Biodiversity decreases with increasing altitude, mainly because of the increasingly adverse climate. In the European Alps, only a few plant species occur above 4,000 m a.s.l., among these is Ranunculus glacialis L. Current studies have shown that R. glacialis has a highly conservative growth strategy and low developmental plasticity in response to different dates of snowmelt. Therefore, it was of particular interest to observe whether this strategy is maintained at higher altitudes and to reveal the reproductive limits. We examined the effect of the date of snowmelt on reproductive development and reproductive success in R. glacialis over several years at two subnival sites (2,650 and 2,880 m a.s.l.) and at a nival site (3,440 m a.s.l.) in the Austrian Alps. At the subnival sites, reproductive performance was relatively stable (prefloration period, i.e. snowmelt to onset of anthesis, 2–3 weeks; postfloration period, i.e. onset of anthesis until fruit maturity, 4–5 weeks). Depending on the date of flowering, the mean seed/ovule (S/O) ratio was 0.5–0.8. The temporal safety margin between seed maturation and the onset of winter conditions was at least 1 month. The situation was quite different in the nival zone: the prefloration period usually lasted 1 month, anthesis up to 2 weeks, and seed development 6–7 weeks; when seeds matured in time, the S/O ratio was 0.4–0.6. Overall, R. glacialis shows a high developmental plasticity. At higher altitudes, R. glacialis can double the time taken for seed development but runs a high risk of seeds not maturing in time.     

Altitudinal divergence in maternal thermoregulatory behaviour may be driven by differences in selection on offspring survival in a viviparous lizard

Plastic responses to temperature during embryonic development are common in ectotherms, but their evolutionary relevance is poorly understood. Using a combination of field and laboratory approaches, we demonstrate altitudinal divergence in the strength of effects of maternal thermal opportunity on offspring birth date and body mass in a live-bearing lizard (Niveoscincus ocellatus). Poor thermal opportunity decreased birth weight at low altitudes where selection on body mass was negligible. In contrast, there was no effect of maternal thermal opportunity on body mass at high altitudes where natural selection favored heavy offspring. The weaker effect of poor maternal thermal opportunity on offspring development at high altitude was accompanied by a more active thermoregulation and higher body temperature in highland females. This may suggest that passive effects of temperature on embryonic development have resulted in evolution of adaptive behavioral compensation for poor thermal opportunity at high altitudes, but that direct effects of maternal thermal environment are maintained at low altitudes because they are not selected against. More generally, we suggest that phenotypic effects of maternal thermal opportunity or incubation temperature in reptiles will most commonly reflect weak selection for canalization or selection on maternal strategies rather than adaptive plasticity to match postnatal environments.     

Altitudinal variation in life cycle syndromes of California populations of the grasshopper,Melanoplus sanguinipes (F.)

Summary Life cycles of California populations of the grasshopper, Melanoplus sanguinipes, varied along an altitudinal gradient. Temperature records indicate a longer season at low altitude on the coast, based on computation of degree days available for development, even though summer air temperatures are cooler than at high altitude; this is a result of warm soil temperatures. At high and low altitudes there was a high proportion of diapause eggs oviposited, while intermediate proportions of diapause eggs occurred at mid altitudes. The low altitude, and especially sea level, populations diapaused at all stages of embryonic development, while at high altitudes most diapause occurred in the late stages just before hatch. Diapause was more intense at high altitudes. One result of diapause differences was delayed hatching in the sea level population. Nymphal development and development of adults to age at first reproduction were both accelerated at high altitude relative to sea level. At lower temperatures (27° C) there was a tendency for short days to accelerate development of sea level nymphs, but not high altitude nymphs. In both sea level and high altitude grasshoppers, short days accelerated maturation of adults to onset of oviposition at warm temperature (33° C); there was little reproduction at 27° C. Population differences for all traits studied appear to be largely genetic with some maternal effects possible. We interpret diapause variation at low and mid altitudes to be responses to environmental uncertainty and variations in development rates to be adaptations to prevailing season lengths.     

Female reproductive cycles in two subspecies of the tropical lizard Mabuya striata

Summary Female reproductive cycles were examined in two subspecies of the live-bearing lizard Mabuya striata from Central Africa, an area with distinct rainy and dry seasons. The low altitude M. s. striata was reproductive throughout the year apart from a brief period at the start of the rainy season, and probably produced three clutches a year. Most females of the high altitude M. s. punctatissima were non-reproductive in the rainy season, came into reproductive condition in the early dry season, and gave birth in the late dry season. For some females, there was a second reproductive cycle starting in the late dry season with birth in the rainy season. For the low altitude M. s. striata it appears that availability of moisture may affect reproduction. For the high altitude M. s. punctatissima it appears that the low temperatures and short hours of sunshine of the early dry season constrain reproduction, and that reproductive cycles are timed to avoid birth occurring in the early dry season.     

Plasticity in reproduction and growth among 52 range‐wide populations of a Mediterranean conifer: adaptive responses to environmental stress

A plastic response towards enhanced reproduction is expected in stressful environments, but it is assumed to trade off against vegetative growth and efficiency in the use of available resources deployed in reproduction [reproductive efficiency (RE)]. Evidence supporting this expectation is scarce for plants, particularly for long‐lived species. Forest trees such as Mediterranean pines provide ideal models to study the adaptive value of allocation to reproduction vs. vegetative growth given their among‐population differentiation for adaptive traits and their remarkable capacity to cope with dry and low‐fertility environments. We studied 52 range‐wide Pinus halepensis populations planted into two environmentally contrasting sites during their initial reproductive stage. We investigated the effect of site, population and their interaction on vegetative growth, threshold size for female reproduction, reproductive–vegetative size relationships and RE. We quantified correlations among traits and environmental variables to identify allocation trade‐offs and ecotypic trends. Genetic variation for plasticity was high for vegetative growth, whereas it was nonsignificant for reproduction. Size‐corrected reproduction was enhanced in the more stressful site supporting the expectation for adverse conditions to elicit plastic responses in reproductive allometry. However, RE was unrelated with early reproductive investment. Our results followed theoretical predictions and support that phenotypic plasticity for reproduction is adaptive under stressful environments. Considering expectations of increased drought in the Mediterranean, we hypothesize that phenotypic plasticity together with natural selection on reproductive traits will play a relevant role in the future adaptation of forest tree species.     

Local adaptation and cogradient selection in the alpine plant, Poa hiemata, along a narrow altitudinal gradient

Alpine environments are particularly susceptible to environmental changes associated with global warming but there is potential for alpine plants to adapt to warming if local adaptation occurs and gene flow allows genotypes adapted to low altitudes to colonize higher altitude sites. Here we examine the adaptive potential of a common alpine grass, Poa hiemata, within the restricted alpine habitat of Australian mountains, across a narrow altitudinal gradient replicated in three areas. Grasses at high altitude sites had shorter leaf lengths and larger circumferences than those at lower sites. Transplant experiments with clonal material and plants grown from seed indicated that these differences were partly genetic, with environmental and genetic factors both contributing to the differences between altitudes. Differences in altitudinal forms were also evident in a common garden experiment. Plants showed a home-site advantage in terms of survival. A fitness analysis indicated that at high altitude sites, selection favored plants with short leaves and larger circumferences, whereas these traits were selected in the opposite direction at the low altitude sites. These findings indicate cogradient selection and potential for both plastic and genotypic shifts in response to climate change in P. hiemata.     

Evidence of adaptive divergence in plasticity: density- and site-dependent selection on shade-avoidance responses in Impatiens capensis

We investigated the conditions under which plastic responses to density are adaptive in natural populations of Impatiens capensis and determined whether plasticity has evolved differently in different selective environments. Previous studies showed that a population that evolved in a sunny site exhibited greater plasticity in response to density than did a population that evolved in a woodland site. Using replicate inbred lines in a reciprocal transplant that included a density manipulation, we asked whether such population differentiation was consistent with the hypothesis of adaptive divergence. We hypothesized that plasticity would be more strongly favored in the sunny site than in the woodland site; consequently, we predicted that selection would be more strongly density dependent in the sunny site, favoring the phenotype that was expressed at each density. Selection on internode length and flowering date was consistent with the hypothesis of adaptive divergence in plasticity. Few costs or benefits of plasticity were detected independently from the expressed phenotype, so plasticity was selected primarily through selection on the phenotype. Correlations between phenotypes and their plasticity varied with the environment and would cause indirect selection on plasticity to be environment dependent. We showed that an appropriate plastic response even to a rare environment can greatly increase genotypic fitness when that environment is favorable. Selection on the measured characters contributed to local adaptation and fully accounted for fitness differences between populations in all treatments except the woodland site at natural density.     

Is seasonal variation in leaf traits adaptive for the annual plant Dicerandra linearifolia?

Empirical studies of phenotypic plasticity have often relied on the plausibility that a plastic response to the environment would increase fitness in order to diagnose the response as adaptive. I conducted a test of the hypothesis that seasonal variation in leaf traits is an adaptive response to seasonal variation in environmental conditions faced by the annual plant Dicerandralinearifolia. This species exhibits variation in leaf morphology and anatomy in response to temperature that is consistent with the expectations for adaptive plasticity. I examined variation in the size, thickness and density of stomata of leaves that develop in summer and winter and used analysis of phenotypic selection during winter and summer seasons to test the hypothesis that seasonal variation in these traits is adaptive. Regression analyses of estimated dry mass (as a proxy for fitness) on leaf traits revealed no evidence supporting the adaptive hypothesis. Selection favoured individuals with large and thick leaves in both winter and summer, and density of stomata had little or no effect on estimated relative fitness in any season. Correspondence between seasonal variation in leaf thickness and density of stomata and expectations for adaptive plasticity appears to be purely fortuitous. Seasonal variation in leaf traits may persist simply because there is no selection against individuals in which these traits vary. My results underscore the importance of definitive tests of the hypothesis of adaptation to distinguish adaptive plasticity from neutral or nonadaptive phenotypic plasticity.     

Changes in reproductive investment with altitude in an alpine plant

Aims In perennial species, the allocation of resources to reproduction results in a reduction of allocation to vegetative growth and, therefore, impacts future reproductive success. As a consequence, variation in this trade-off is among the most important driving forces in the life-history evolution of perennial plants and can lead to locally adapted genotypes. In addition to genetic variation, phenotypic plasticity might also contribute to local adaptation of plants to local conditions by mediating changes in reproductive allocation. Knowledge on the importance of genetic and environmental effects on the trade-off between reproduction and vegetative growth is therefore essential to understand how plants may respond to environmental changes.Methods We conducted a transplant experiment along an altitudinal gradient from 425 to 1?921 m in the front range of the Western Alps of Switzerland to assess the influence of both altitudinal origin of populations and altitude of growing site on growth, reproductive investment and local adaptation in Poa alpina .Important findings In our study, the investment in reproduction increased with plant size. Plant growth and the relative importance of reproductive investment decreased in populations originating from higher altitudes compared to populations originating from lower altitudes. The changes in reproductive investment were mainly explained by differences in plant size. In contrast to genetic effects, phenotypic plasticity of all traits measured was low and not related to altitude. As a result, the population from the lowest altitude of origin performed best at all sites. Our results indicate that in P. alpina genetic differences in growth and reproductive investment are related to local conditions affecting growth, i.e. interspecific competition and soil moisture content.     

Maintenance of sexually dimorphic preadult traits in Marchantia inflexa (Marchantiacae)

Marchantia inflexa, a dioecious thallose liverwort, is sexually dimorphic in clonal expansion traits. We used selection analyses to measure the magnitude and direction of selection on clonal fitness to uncover possible mechanisms for the maintenance of preadult sexually dimorphic characters. We planted replicates of genotypes of female and male M. inflexa in two light environments in a greenhouse and measured morphological and phenological characters associated with growth and asexual reproduction. Timing to onset of asexual reproduction and plant size early in development were under sex-specific selection in a low light environment. Additionally, females exhibited a sex-specific cost of plasticity in the timing of their onset of asexual reproduction in high light. Selection on asexual fitness tended to shift traits toward monomorphism rather than sexual dimorphism, whereas the expressed phenotype of females was congruent with patterns of selection acting on sexual fitness. We detected negative trade-offs between asexual and sexual fitness components in females in one light environment. Opposing selective forces acting on asexual and sexual fitness components may explain how sexual dimorphisms persist in the face of selection for monomorphism in the preadult phase.     

Effects of four generations of density-dependent selection on life history traits and their plasticity in a clonally propagated plant

Life history evolution of many clonal plants takes place with long periods of exclusively clonal reproduction and under largely varying ramet densities resulting from clonal reproduction. We asked whether life history traits of the clonal herb Ranunculus reptans respond to density-dependent selection, and whether plasticity in these traits is adaptive. After four generations of exclusively clonal propagation of 16 low and 16 high ramet-density lines, we studied life history traits and their plasticities at two test ramet-densities. Plastic responses to higher test-density consisted of a shift from sexual to vegetative reproduction, and reduced flower production, plant size, branching frequency, and lengths of leaves and internodes. Plants of high-density lines tended to have longer leaves, and under high test-density branched less frequently than those of low-density lines. Directions of these selection responses indicate that the observed plastic branching response is adaptive, whereas the plastic leaf length response is not. The reverse branching frequency pattern at low test-density, where plants of high-density lines branched more frequently than those of low-density lines, indicates evolution of plasticity in branching. Moreover, when grown under less stressful low test-density, plants of high-density lines tended to grow larger than the ones of low-density lines. We conclude that ramet density affects clonal life-history evolution and that under exclusively clonal propagation clonal life-history traits and their plasticities evolve differently at different ramet densities.     

Reproductive strategies in two arctic Pedicularis species (Scrophulariaceae)

A study of a number of reproductive traits in two sympatric species of Pedicularis in northern Swedish Lapland, the subarctic-alpine P lapponica and the artic P hursuta , revealed that the life-history strategies of the two species differ profoundly High fruit set and low seed abortion rate, as m P hursuta , is common in arctic plants in late-thawing habitats and represents a case of extreme adversity selection rather than an indication of a ruderal life-history strategy Pedicularis lapponica , on the other hand, is a typical K-strategist (or stress-tolerator) requiring a longer period of growth for optimal reproduction Occuring at both low and high altitudes in the area, P lapponica tends to increase in self-compatibility with altitude, which is interpreted as an adaptation to lower pollinator visitation frequency in arctic environments The variation in length of the protruding part of the style in P lapponica is shown to be correlated with exposure to light Predispersal seed predation is severe m P lapponica at low altitudes, where the capsules are attacked by fly and moth larvae At high altitudes, a minor proportion of the capsules of P lapponica experience predation and only from flies, while P hursuta is completely unpredated     

The Response of the Alpine Dwarf Shrub Salix herbacea to Altered Snowmelt Timing: Lessons from a Multi-Site Transplant Experiment

Climate change is altering spring snowmelt patterns in alpine and arctic ecosystems, and these changes may alter plant phenology, growth and reproduction. To predict how alpine plants respond to shifts in snowmelt timing, we need to understand trait plasticity, its effects on growth and reproduction, and the degree to which plants experience a home-site advantage. We tested how the common, long-lived dwarf shrub Salix herbacea responded to changing spring snowmelt time by reciprocally transplanting turfs of S. herbacea between early-exposure ridge and late-exposure snowbed microhabitats. After the transplant, we monitored phenological, morphological and fitness traits, as well as leaf damage, during two growing seasons. Salix herbacea leafed out earlier, but had a longer development time and produced smaller leaves on ridges relative to snowbeds. Longer phenological development times and smaller leaves were associated with reduced sexual reproduction on ridges. On snowbeds, larger leaves and intermediate development times were associated with increased clonal reproduction. Clonal and sexual reproduction showed no response to altered snowmelt time. We found no home-site advantage in terms of sexual and clonal reproduction. Leaf damage probability depended on snowmelt and thus exposure period, but had no short-term effect on fitness traits. We conclude that the studied populations of S. herbacea can respond to shifts in snowmelt by plastic changes in phenology and leaf size, while maintaining levels of clonal and sexual reproduction. The lack of a home-site advantage suggests that S. herbacea may not be adapted to different microhabitats. The studied populations are thus unlikely to react to climate change by rapid adaptation, but their responses will also not be constrained by small-scale local adaptation. In the short term, snowbed plants may persist due to high stem densities. However, in the long term, reduction in leaf size and flowering, a longer phenological development time and increased exposure to damage may decrease overall performance of S. herbacea under earlier snowmelt.     

Gas Exchange and Water Use Efficiency of Adenophora lobophylla (Campanulaceae) at Different Altitudes on the East Boundary of the Qing- Zang Plateau

Field measurements were made of leaf net photosynthetic rate (Pn), apparent quantum yield (AQY), dark respiration (Rd), transpiration (Tr), water use efficiency (WUE), stomatal conductance (Gs), intercellular CO2 concentration (Ci), leaf temperature (TI), stomatal density, air temperature (Ta) and relative soil water content (SWC) for Adenophora lobophylla Hong in August, 1996. The species grows in the region from an altitude of 2 300 m to 3 400 m on the eastern boundary of the Qing-Zang Plateau in Sichuan Province, China. Leaf gas exchange, water use efficiency and plasticity of populations in field were compared among different altitudes to evaluate the possible interactions between adaptation of A. lobophylla and environmental factors in these habitats. Pn and AQY at low altitudes were lower than those at high altitudes. They strongly responded to SWC and Ta. On the other hand, Rd at low altitudes was higher than that at high altitudes because of the higher air temperature there. The growth rates at low altitudes were associated with the increases in Rd and a relevant less Pn. Stomata showed strong responses to leaf-to-air vapor pressure deficit at the leaf surface (Vpdl) and Tl in these habitats. Increasing stomatal limitations to photosynthesis appeared to be responsible for the reduction in Pn at high Tl, Vpdl and low available soil water for A. lobophylla at low ahitudes. Nonstomatal limitation to photosynthesis also happened at extreme soil water deficits and high Tl and Vpdl at an altitude of 2 300 m. Tr had a close relationship with stomatal conductance and was also affected by leaf temperature and leaf-to-air vapor pressure deficit at the leaf surface among habitats grown in different ahitudes. WUE increased with altitude. Increasing stomatal densities showed different plasticity of A. lobophylla as altitude increased. SWC and Ta appeared to be important factors to limit carbon assimilation in A. lobophylla at low altitudes, primarily through the process of stomatal closure. The overall results are in consistence with the hypothesis that strong pressure from tmfavorable environmental factors to gas exchange and wateruse of A. lobophylla may prevent their population expansion at low altitudes, which indicate that the above-mentioned restrictions might lead to the endangerment of A. lobophylla.     

Plasticity to light cues and resources in Arabidopsis thaliana: testing for adaptive value and costs

Plants shaded by neighbors or overhead foliage experience both a reduction in the ratio of red to far red light (R:FR), a specific cue perceived by phytochrome, and reduced photosynthetically active radiation (PAR), an essential resource. We tested the adaptive value of plasticity to crowding and to the cue and resource components of foliage shade in the annual plant Arabidopsis thaliana by exposing 36 inbred families from four natural populations to four experimental treatments: (1) high density, full sun; (2) low density, full sun; (3) low density, neutral shade; and (4) low density, low R:FR-simulated foliage shade. Genotypic selection analysis within each treatment revealed strong environmental differences in selection on plastic life-history traits. We used specific contrasts to measure plasticity to density and foliage shade, to partition responses to foliage shade into phytochrome-mediated responses to the R:FR cue and responses to PAR, and to test whether plasticity was adaptive (i.e., in the same direction as selection in each environment). Contrary to expectation, we found no evidence for adaptive plasticity to density. However, we observed both adaptive and maladaptive responses to foliage shade. In general, phytochrome-mediated plasticity to the R:FR cue of foliage shade was adaptive and counteracted maladaptive growth responses to reduced PAR. These results support the prediction that active developmental responses to environmental cues are more likely to be adaptive than are passive resource-mediated responses. Multiple regression analysis detected a few costs of adaptive plasticity and adaptive homeostasis, but such costs were infrequent and their expression depended on the environment. Thus, costs of plasticity may occasionally constrain the evolution of adaptive responses to foliage shade in Arabidopsis, but this constraint may differ among environments and is far from ubiquitous.     

The relative advantages of plasticity and fixity in different environments: when is it good for a plant to adjust?

Plant populations and species differ greatly in phenotypic plasticity. This could be because plasticity is advantageous under some conditions and disadvantageous or not advantageous under others. We distinguish adaptive from injurious and neutral plasticity and discuss when selection should favor adaptive plasticity over genetic differentiation or lack of phenotypic variation. It seems reasonable to hypothesize that selection is likely to favor plasticity when an environmental factor varies on the same spatial scale as the plant response unit, when the plant can respond to an environmental factor faster than the level of the factor changes, and when environmental variation is highly but not completely predictable. Phenotypic plasticity might also tend to be more advantageous when mean resource availability is high rather than low, when a response can occur late in development rather than early, and when a response is reversible rather than irreversible. There is substantial evidence for the hypothesis that predictability favors plasticity. However, available evidence does not support the hypothesis that high mean resource availability necessarily favors plasticity. Testing hypotheses about when it is good for a plant to adjust is central to understanding the diversity of plasticity in plants.     

Seed Dispersal as a Maternally Influenced Character: Mechanistic Basis of Maternal Effects and Selection on Maternal Characters in an Annual Plant

Maternal influences on progeny characters affect phenotypic correlations between characters expressed in maternal and progeny generations and consequently influence evolutionary responses to selection. Net selection on maternally influenced characters depends on selection both on the progeny character and on the maternal characters that influence it. I used seed dispersal in Cakile edentula as a system in which to identify the mechanisms of environmentally mediated maternal effects and to determine how selection on maternal characters alters the adaptive value of dispersal. In C. edentula, maternal morphology responds to conspecific density experienced by the mother. Maternal morphology in turn affects offspring (seed) dispersal and density and thereby offspring morphology and fitness. I estimated the magnitude of density-mediated maternal effects on dispersal and identified their mechanism by characterizing the plasticity of maternal morphology to density. I also measured density-dependent selection on maternal characters that influence dispersal. Maternal plasticity to density was caused by both allometric and nonallometric variation in morphology, and this plasticity resulted in a negative correlation between maternal and progeny density. Such negative maternal effects are expected to retard responses to selection. Maternal morphology influenced maternal fitness, in part through the relationship of fitness to maternal plant size and in part through size-independent fitness effects. Maternal phenotypes that promote dispersal, and thereby increase progeny fitness, were associated with decreased maternal fitness. Selection on dispersal at the level of progeny favors increased dispersal; maternal influences on dispersal, however, not only cause a greatly reduced adaptive value of dispersal but lead to the prediction of a slower response to selection.     

Genetic improvement of production while maintaining fitness

Selection for production tends to decrease fitness, in particular, major components such as reproductive performance. Under an infinitesimal genetic model restricted index selection can maintain reproductive performance while improving production. However, reproductive traits are thought to be controlled by a finite number of recessive alleles at low frequency. Culling for low reproduction may weed out the negative homozygous genotypes for reproduction in any generation, thus controlling the frequencies of alleles negative for reproduction. Restricted index selection, culling for low reproduction and a new method called empirical restricted index selection were compared for their efficiency in improving production while maintaining reproduction. Empirical restricted index selection selects animals that have on average the highest estimated breeding values for production and on average the same estimated breeding values for reproduction as the base population. An infinitesimal genetic model and models with a finite number of loci for reproduction with rare deleterious recessive alleles, which have additive, dominant or no pleiotropic effects on production, were considered. When reproduction was controlled by a finite number of loci with rare recessive alleles, restricted index selection could not maintain reproduction. The culling of 20% of the animals on reproduction maintained reproduction with all genetic models, except for the model where loci for reproduction had additive effects on production. Empirical restricted selection maintained reproduction with all models and yielded higher production responses than culling on reproduction, except when there were dominant pleiotropic effects on production.     

Phenological responses of upland rice grown along an altitudinal gradient

High altitude upland rice (Oryza sativa L.) production systems are expected to benefit from climate change induced increase in temperatures. The potential yield of rice genotypes is governed by the thermal environment experienced during crop development phases when yield components are determined. Thus, knowledge on genotypic variability in phenotypic responses to variable temperature is required for assessing the adaptability of rice production to changing climate. Although, several crop models are available for this task, genotypic thermal constants used to simulate crop phenology vary strongly among the models and are under debate. Therefore, we conducted field trials with ten contrasting upland rice (O. sativa L.) genotypes on three locations along an altitudinal gradient with five monthly staggered sowing dates for two years in Madagascar with the aim to study phenological responses at different temperature regimes. We found that, crop duration is equally influenced by genotype selection, sowing date and year in the high altitude. In contrast, in mid altitudes genotype has no effect on crop duration. At low altitudes crop duration is more affected by sowing date. Grain yield is strongly affected by low temperatures at high altitudes and severly influenced by frequent tropical cyclones at low altitudes. In high altitude, genotype explained 68% of variation in spikelet sterility, whereas in mid and low altitudes environment explained more than 70% of the variation. The phenological responses determining crop duration and yield, the basic genotypic thermal constants, and the analyses of genotypic thermal responses with regard to spikelet sterility reported here, provide valuable information for the improvement of rice phenological models urgently needed to develop new genotypes and better adapted cropping calendars.     

Altitude, migration, and fertility in the Andes

In order to examine the relationship between hypoxia and reduced fertility of high Andean populations, a sample of 241 females living in the low-altitude Tambo Valley of Peru was studied. 63 of the subjects were born in the low-altitude valley, 121 were migrants from high altitudes, and 57 were migrants born in low altitudes. The rate of abortion was low among high-altitude subjects before they migrated, but became greater after migrating. It was found that the high-altitude populations had almost twice as long parity intervals than the low-altitude populations. Compared to migrants born at low altitudes, the high-altitude-born subjects who migrated to low altitudes had higher fertility rates. The results of the study are consistent with the hypothesis that high altitudes, through anoxia, have a lowering effect on fertility. Of the several possible explanations which might account for the increase in fertility of downward migrants on migration from high to low altitude (migration, socioeconomic factors, acculturation, seasonal male emigration from high altitude, and removal of hypoxia stress), altitude appears to be the most significant.