Ecotype Formation Research Articles

Are plant taxa found on the Witwatersrand serpentine ecotypes or substrate-generalists?

Serpentine soils vary in levels of toxicity of the parent material, climate, relief, and biological activity resulting in a range of harsh environments. This harsh environment is likely to result in a unique, usually endemic, flora. However, not all plants growing on serpentine soils are endemic; some plants appear to be indifferent (bodenvag) to the serpentine condition. It is likely that these apparently bodenvag serpentine taxa may be ecotypically differentiated into serpentine tolerant and intolerant races whilst appearing morphologically indistinguishable (cryptic). This study investigated whether aspects of life history (biomass, productivity, life forms) and breeding system characteristics (pollen viability, pollen/ovule ratios, pollinator exclusions, pollination syndromes) could indicate ecotype formation in selected, apparently bodenvag, serpentine taxa on the Witwatersrand. Multivariate analyses, namely correspondence analysis (CA) and principal component analysis (PCA) as well as Student's t-tests, were used to analyse the data. The results indicate that reproductive and life history data do not separate the study taxa into ecotypes. This study concludes that the relatively mild serpentine condition on the Witwatersrand does not exert a strong influence on plants and that there is still likely to be gene flow amongst the study taxa growing on and off serpentine soils.

Repeated evolution of reproductive isolation in a marine snail: unveiling mechanisms of speciation

Distinct ecotypes of the snail Littorina saxatilis, each linked to a specific shore microhabitat, form a mosaic-like pattern with narrow hybrid zones in between, over which gene flow is 10-30% of within-ecotype gene flow. Multi-locus comparisons cluster populations by geographic affinity independent of ecotype, while loci under selection group populations by ecotype. The repeated occurrence of partially reproductively isolated ecotypes and the conflicting patterns in neutral and selected genes can either be explained by separation in allopatry followed by secondary overlap and extensive introgression that homogenizes neutral differences evolved under allopatry, or by repeated evolution in parapatry, or in sympatry, with the same ecotypes appearing in each local site. Data from Spain, the UK and Sweden give stronger support for a non-allopatric model of ecotype formation than for an allopatric model. Several different non-allopatric mechanisms can, however, explain the repeated evolution of the ecotypes: (i) parallel evolution by new mutations in different populations; (ii) evolution from standing genetic variation; and (iii) evolution in concert with rapid spread of new positive mutations among populations inhabiting similar environments. These models make different predictions that can be tested using comprehensive phylogenetic information combined with candidate loci sequencing.

Case studies and mathematical models of ecological speciation. 3: Ecotype formation in a Swedish snail

Formation of partially reproductively isolated ecotypes in the rough periwinkle, Littorina saxatilis, may be a case of incipient nonallopatric ecological speciation. To better understand the dynamics of ecotype formation, its timescale, driving forces and evolutionary consequences, we developed a spatially explicit, individual-based model incorporating relevant ecological, spatial and mate selection data for Swedish L. saxatilis. We explore the impact of bounded hybrid superiority, ecological scenarios and mate selection systems on ecotype formation, gene flow and the evolution of prezygotic isolation. Our model shows that ecotypes are expected to form rapidly in parapatry under conditions applicable to Swedish L. saxatilis and may proceed to speciation. However, evolution of nonrandom mating had complex behaviour. Ecotype evolution was inhibited by pre-existing mating preferences, but facilitated by the evolution of novel preferences. While in many scenarios positive assortative mating reduced gene flow between ecotypes, in others negative assortative mating arose, preferences were lost after ecotype formation, preferences were confined to one ecotype or the ancestral ecotype became extinct through sexual selection. Bounded hybrid superiority (as observed in nature) enhanced ecotype formation but increased gene flow. Our results highlight that ecotype formation and speciation are distinct processes: factors that contribute to ecotype formation can be detrimental to speciation and vice versa. The complex interactions observed between local adaptation and nonrandom mating imply that generalization from data is unreliable without quantitative theory for speciation.

Stress gradients select for ecotype formation in Cicer judaicum Boiss., a wild relative of domesticated chickpea

The investigation of plant adaptive strategies has been enhanced by the advent of high resolution climate models facilitating greatly improved fine-scale habitat characterization. We have used this approach in the evaluation of C. judaicum Boiss., an annual wild relative of chickpea. 54 accessions from 12 Israeli populations representing three separate habitats differing in temperature and terminal drought stress intensity were evaluated in a common garden experiment, measuring phenology and growth. The results indicate that C. judaicum formed distinct ecotypes along environmental stress gradients, with stress avoidance as a key adaptive strategy: (1) germination is delayed with increasing collection site altitude and associated decreasing temperatures; (2) flowering date and productivity (as estimated by mainstem length) are inversely related to habitat stress, as defined by site climate or soil type. Populations from stressful sites escape drought stress through early flowering at the likely cost of biomass production. We conclude that precise habitat characterization facilitates the study of specific adaptation over relatively short geographic distances. This is particularly pertinent today because many of the key crop production system stresses (i.e., terminal drought, the impact of climate change) can be modelled climatically to identify potentially-adapted germplasm from ex situ and in situ collections.

Ecological Niche Dimensionality and the Evolutionary Diversification of Stick Insects

The degree of phenotypic divergence and reproductive isolation between taxon pairs can vary quantitatively, and often increases as evolutionary divergence proceeds through various stages, from polymorphism to population differentiation, ecotype and race formation, speciation, and post-speciational divergence. Although divergent natural selection promotes divergence, it does not always result in strong differentiation. For example, divergent selection can fail to complete speciation, and distinct species pairs sometimes collapse (‘speciation in reverse’). Widely-discussed explanations for this variability concern genetic architecture, and the geographic arrangement of populations. A less-explored possibility is that the degree of phenotypic and reproductive divergence between taxon pairs is positively related to the number of ecological niche dimensions (i.e., traits) subject to divergent selection. Some data supporting this idea stem from laboratory experimental evolution studies using Drosophila, but tests from nature are lacking. Here we report results from manipulative field experiments in natural populations of herbivorous Timema stick insects that are consistent with this ‘niche dimensionality’ hypothesis. In such insects, divergent selection between host plants might occur for cryptic colouration (camouflage to evade visual predation), physiology (to detoxify plant chemicals), or both of these niche dimensions. We show that divergent selection on the single niche dimension of cryptic colouration can result in ecotype formation and intermediate levels of phenotypic and reproductive divergence between populations feeding on different hosts. However, greater divergence between a species pair involved divergent selection on both niche dimensions. Although further replication of the trends reported here is required, the results suggest that dimensionality of selection may complement genetic and geographic explanations for the degree of diversification in nature.

Nonallopatric and parallel origin of local reproductive barriers between two snail ecotypes.

Theory suggests that speciation is possible without physical isolation of populations (hereafter, nonallopatric speciation), but recent nonallopatric models need the support of irrefutable empirical examples. We collected snails (Littorina saxatilis) from three areas on the NW coast of Spain to investigate the population genetic structure of two ecotypes. Earlier studies suggest that these ecotypes may represent incipient species: a large, thick-shelled 'RB' ecotype living among the barnacles in the upper intertidal zone and a small, thin-shelled 'SU' ecotype living among the mussels in the lower intertidal zone only 10-30 m away. The two ecotypes overlap and hybridize in a midshore zone only 1-3 m wide. Three different types of molecular markers [allozymes, mitochondrial DNA (mtDNA) and microsatellites] consistently indicated partial reproductive isolation between the RB and the SU ecotypes at a particular site. However, each ecotype was related more closely to the other ecotype from the same site than to the same ecotype from another site further along the Galician coast (25-77 km away). These findings supported earlier results based solely on allozyme variation and we could now reject the possibility that selection produced these patterns. The patterns of genetic variation supported a nonallopatric model in which the ecotypes are formed independently at each site by parallel evolution and where the reproductive barriers are a byproduct of divergent selection for body size. We argue that neither our laboratory hybridization experiments nor our molecular data are compatible with a model based on allopatric ecotype formation, secondary overlap and introgression.

Variações fenotípicas e potencial plástico de Eugenia calycina Cambess. (Myrtaceae) em uma área de transição cerrado-vereda

The aims of this work were to verify the incidence of phenotypic variation and the plastic potential of Eugenia calycina in a transitional savanna/palm swamp area. Subpopulations were characterized by seed weight, number of seeds per fruit and number of flowers per plant. Moreover, density of occurrence and the rate of parasitism on fruits by larvae of flies (Diptera: Tephritidae) in each area were significantly different. Asynchronism of the flowering period, being late for the savanna area, was also observed. A genetic quantitative model was used to measure phenotypic plasticity based on a reciprocal transplantation experiment of sibling-seeds into soils collected from each area. Three aspects of plastic potential were evaluated: height of steam and number and length of the seedlings leaves. Observed differences in these characters were attributed not only to genetic divergences but also to phenotypic plasticity. The individuals showed different plastic responses, diverging in their ability to respond to environmental influences. The data confirm the role of phenotypic plasticity in the adaptive and evolutionary process involved with ecotype formation in savanna/palm swamp transitional vegetation.

Evolution in Littorina: ecology matters

Organisms of marine rocky shores are exposed to physical stress from abiotic factors, such as temperature, salinity and wave action. These factors vary over compressed temporal and spatial scales, producing an exceedingly heterogeneous habitat with steep gradients of selection, and it seems likely that this has a strong influence on the evolution of populations of rocky shore organisms. With the periwinkles (genus Littorina) as a model group, I review strategies for coping with small-scale heterogeneous environments and what implications these strategies have on the evolution of these species. Some species of Littorina have long-lived pelagic larvae and sites of various habitats are thus recruited from a common gene pool. This largely prevents local adaptation but minor adjustments are possible through a plastic phenotype. Other species of the genus are directly developing with no larval dispersal and among these there is evidence of strong local adaptation forming distinct ecotypes in contrasting habitats by parallel evolution. In at least one of the directly developing species ( L. saxatilis) divergent selection among ecotypes has resulted in partial reproductive barriers that further impede gene flow among ecotypes. Furthermore, convergent evolution among species has produced superficially similar morphs in different habitats. Ecotype formation, ecological reproductive barriers and convergence among species all indicate that ecological processes are critical for evolution of Littorina species.

Patterns of allozyme and morphological differentiation in the Carex flava complex (Cyperaceae) in Fennoscandiaxs

Allozyme and morphological differentiation patterns within the Carex flava complex in Fennoscandia were described by means of multivariate analyses. As a result of these studies, and considering also data on ecology, distribution and hybridization, the following taxa were recognized: C. flava with no infraspecific taxa, C. lepidocarpa with two subspecies, ssp. lepidocarpa and ssp. jemtlandica, C. demissa with no infraspecific taxa, and C. oederi with three varieties, var. oederi, var. bergrothii, and var. pulchella. Carex lepidocarpa ssp. jemtlandica was allozymically divergent from ssp. lepidocarpa, indicating a long period of isolation of the two subspecies. A poor morphological separation in some localities is apparently due to secondary contacts in connection with recolonization of Fennoscandia after the last ice age, resulting in the formation of fertile hybrids and back‐crosses. The subdivision of C. oederi into three varieties is supported by ecological data and it is suggested that they have originated by parallel ecotype formation under similar selective regimes in different areas. However, the varietal limits are not supported by differentiation in neutral (allozyme) characters. A phylogenetic hypothesis was constructed by means of neighbour‐joining based allozyme data and it was indicated that C. demissa may be more closely related to C. lepidocarpa than to C. oederi. Previous treatments that have reduced the C. flava complex to one or two species may have been based on inappropriate species concepts, too coarse methods of analysis of morphological data, ignorance of patterns of co‐occurrence and hybridization, or a combination of these factors.

Potencial plástico de Cabralea canjerana subsp. polytricha (Adr. Juss.) Penn. (Meliaceae) e seu papel na formação de ecótipos em áreas de cerrado e vereda, Uberlândia, MG

The ecotype formation of Cabralea canjerana subsp. polytricha in areas of savanna and palm swamp in an ecological reserve in Uberlândia, MG, was studied, using a quantitative genetic model to measure phenotypic plasticity. Ecotypes were characterised by the height of the individuals, weight of fruits and seeds, number of viable seeds per fruit and synchronization of the flowering period. There is great genetic variability in important fitness characters of the subspecies. The genotypes respond phenotypically to the environmental heterogeneity produced by the savanna/palm swamp gradient in two of the three measured characters. The plastic responses were distinct among genotypes in both direction and intensity. Asynchrony of the flowering period and the action of seed dispersers may contribute to increase the divergences of the ecotypes in the study area.

Specific Rapd Markers of Imperata Cylindrica Populations in Taiwan

Imperata cylindrical L. Beauv, a common C4 grass, is widely distributed in various habitats in Taiwan. Among 55 populations in various habitats, six sites were selected for the study. These sites are namely, Chuwei (an estuary man-grove forest area with high salinity), Sarlun (in coastal saline area), Kengting (at south tip with dry season in wither), Penghu (an islet about 100 km from west coast and is dry in most seasons), and Nankang and Hoshe (both in in-land with mild weather as control sites). The total DNA was extracted from leaves of I. cylindrical collected from the six sites. The DNA was then analyzed by means of a random amplified polymorphic DNA (RAPD) technique. Fourty primers were used for the RAPD anlysis, and only four primers, namely OPA-16, OPA-20, OPV-03, and OPV-13, were successful in obtaining genetic mark-ers for the populations at the sites. Five markers, namely A162, A163, V32, V33, and V131 were obtained. Of them, markers 162 and 163 were generated from primer OPA-16 was found in populations of Nankang, Penghu and Sarlum; however, it was very specific to population Penghu. Markers A163 was only found in population Sarlun. Markers V32 and V33 were generated from primer OPV-03; the former was present all populations except population Chuwei, while later was only found in the Chuwei pop-ulation. Marker V131 was generated from primer OPV-13 and marker A163 was from OPA-16. The marker V131 was specific to the Chuwei population. Regarding the length of the markers, marker V32 has 1198 bp, marker V33 possesses 1173 bp, and marker V131 reveals 1166 bp. Comparing the DNA se-quence of markers V32 and V33, they exhibited 64% homology. All of these population-specific markers are repetitive sequences as revealed from the RFLP analysis. It is concluded that the aforementioned markers are specific to either inter-or intra-specific population of I. cylindrical. It is also suggested that the unique ecotype formation of Chuwei population is a result of adapta-tion to stressful environment, such as anaerobic and saline habitat.

Adaptation of Growth Form in Echium Leucophaeum (Boraginaceae)

Ecotype formation in eight populations of Echium leucophaeum was studied on La Palma (Canary Islands) at 100—m intervals from sea level to 600 m elevation. Climate varied with altitude, the lower elevation having higher temperatures, evaporation, and wind velocity and lower rainfall. A method for recording growth form of individual plants was developed whereby lateral branch production and branching patterns of woody plants could readily be determined. Height growth, lateral branch production, periodicity, and number of branching cycles of Echium were all found to be significantly different in the field, plants from higher elevations having longer branches, more laterals per cycle, fewer cycles, and retarded flowering and fruiting compared with plants near sea level. Two generations of seedlings of the extreme types grown in transplant gardens showed significant differences in inherent rate of growth, indicating genetic adaptation of high and low altitude ecotypes. No significant differences could be shown in the rate of lateral branch formation and periodicity; differences observed in the field with respect to these characteristics may be physiological responses.

イネカラバエの生態型に関する2・3の観察

イネカラバエの生態型が安定なものかどうかについて2, 3の実験を行ない,つぎの結果を得た。春世代における幼虫の発育は3世代型では速く,2世代型ではおそかった。両型の交雑世代幼虫(F1, F2)の発育は3世代型から2世代型まで連続的に変異したが,発育の変異状態は両親の雌雄の組み合せによって異なり,偏母的な傾向をおびた。つぎに,3世代型(鳥取産)の材料を2世代型の生息地(秋田)で3年間(9世代)にわたり野外で継続飼育を行なったが,この間3世代型固有の生態的性質は失なわれなかった。以上のことから,イネカラバエの2つの生態型は環境依存的ではなく遺伝的に安定したものであると結論された。