Senecio Lautus Research Articles

Gravitropic Gene Expression Divergence Associated With Adaptation to Contrasting Environments in an Australian Wildflower.

Plants adapt to their local environment through complex interactions between genes, gene networks and hormones. Although the impact of gene expression on trait regulation and evolution has been recognised for many decades, its role in the evolution of adaptation is still a subject of intense exploration. We used a Multi-parent Advanced Generation Inter-Cross (MAGIC) population, which we derived from crossing multiple parents from two distinct coastal ecotypes of an Australia wildflower, Senecio lautus. We focused on studying the contrasting gravitropic behaviours of these ecotypes, which have evolved independently multiple times and show strong responses to natural selection in field experiments, emphasising the role of natural selection in their evolution. Here, we investigated how gene expression differences have contributed to the adaptive evolution of gravitropism. We studied gene expression in 60 pools at five time points (30, 60, 120, 240 and 480 min) after rotating half of the pools 90°. We found 428 genes with differential expression in response to the 90° rotation treatment. Of these, 81 genes (~19%) have predicted functions related to the plant hormones auxin and ethylene, which are crucial for the gravitropic response. By combining insights from Arabidopsis mutant studies and analysing our gene networks, we propose a preliminary model to explain the differences in gravitropism between ecotypes. This model suggests that the differences arise from changes in the transport and availability of the two hormones auxin and ethylene. Our findings indicate that the genetic basis of adaptation involves interconnected signalling pathways that work together to give rise to new ecotypes.

Uncovering the genetic architecture of parallel evolution.

Identifying the genetic architecture underlying adaptive traits is exceptionally challenging in natural populations. This is because associations between traits not only mask the targets of selection but also create correlated patterns of genomic divergence that hinder our ability to isolate causal genetic effects. Here, we examine the repeated evolution of components of the auxin pathway that have contributed to the replicated loss of gravitropism (i.e. the ability of a plant to bend in response to gravity) in multiple populations of the Senecio lautus species complex in Australia. We use a powerful approach which combines parallel population genomics with association mapping in a Multiparent Advanced Generation Inter-Cross (MAGIC) population to break down genetic and trait correlations to reveal how adaptive traits evolve during replicated evolution. We sequenced auxin and shoot gravitropism-related gene regions in 80 individuals from six natural populations (three parallel divergence events) and 133 individuals from a MAGIC population derived from two of the recently diverged natural populations. We show that artificial tail selection on gravitropism in the MAGIC population recreates patterns of parallel divergence in the auxin pathway in the natural populations. We reveal a set of 55 auxin gene regions that have evolved repeatedly during the evolution of the species, of which 50 are directly associated with gravitropism divergence in the MAGIC population. Our work creates a strong link between patterns of genomic divergence and trait variation contributing to replicated evolution by natural selection, paving the way to understand the origin and maintenance of adaptations in natural populations.

Phenotypic and genotypic parallel evolution in parapatric ecotypes of Senecio.

The independent and repeated adaptation of populations to similar environments often results in the evolution of similar forms. This phenomenon creates a strong correlation between phenotype and environment and is referred to as parallel evolution. However, we are still largely unaware of the dynamics of parallel evolution, as well as the interplay between phenotype and genotype within natural systems. Here, we examined phenotypic and genotypic parallel evolution in multiple parapatric Dune‐Headland coastal ecotypes of an Australian wildflower, Senecio lautus. We observed a clear trait‐environment association in the system, with all replicate populations having evolved along the same phenotypic evolutionary trajectory. Similar phenotypes have arisen via mutational changes occurring in different genes, although many share the same biological functions. Our results shed light on how replicated adaptation manifests at the phenotypic and genotypic levels within populations, and highlight S. lautus as one of the most striking cases of phenotypic parallel evolution in nature.

Highly Replicated Evolution of Parapatric Ecotypes.

Parallel evolution of ecotypes occurs when selection independently drives the evolution of similar traits across similar environments. The multiple origins of ecotypes are often inferred based on a phylogeny that clusters populations according to geographic location and not by the environment they occupy. However, the use of phylogenies to infer parallel evolution in closely related populations is problematic because gene flow and incomplete lineage sorting can uncouple the genetic structure at neutral markers from the colonization history of populations. Here, we demonstrate multiple origins within ecotypes of an Australian wildflower, Senecio lautus. We observed strong genetic structure as well as phylogenetic clustering by geography and show that this is unlikely due to gene flow between parapatric ecotypes, which was surprisingly low. We further confirm this analytically by demonstrating that phylogenetic distortion due to gene flow often requires higher levels of migration than those observed in S. lautus. Our results imply that selection can repeatedly create similar phenotypes despite the perceived homogenizing effects of gene flow.

Taxonomic realignment of Senecio glaucophyllus (Asteraceae; Senecioneae) necessitates a new name for a widespread New Zealand species

ABSTRACT As presently delimited, the endemic New Zealand species Senecio glaucophyllus Cheeseman shows considerable morphological and ecological diversity. Two morphologically similar forms associated with this taxon are found in northwest Nelson (South Island), of which one is often filed in herbaria as S. glaucophyllus subsp. glaucophyllus. The other form has been informally recognised as S. aff. glaucophyllus. The present study aimed to determine if S. aff. glaucophyllus merits taxonomic recognition by assessing if it is morphologically and genetically distinct from S. glaucophyllus subsp. glaucophyllus and, if so, to identify its diagnostic morphological characters. The results of Principal Coordinate Analyses of a morphometric data set and phylogenetic studies of an ITS DNA sequence data set show that the two taxa are morphologically and genetically distinct and only distantly related to each other. Random Forest analyses resulted in the discovery of several morphological characters that can be used to distinguish the taxa. The inclusion of data of type specimens of S. glaucophyllus in the PCoAs showed that specimens of S. aff. glaucophyllus are conspecific with S. glaucophyllus whereas most of S. glaucophyllus in its current delimitation belong to a taxon that is unnamed at the species level. We here provide a new name at species rank for this taxon: Senecio matatini. This taxonomic realignment reinstates Cheeseman’s original concept of S. glaucophyllus as a species that is restricted to northwest Nelson. Senecio matatini has a much wider distribution and is found throughout much of the South Island and the southern and central parts of the North Island. Because our results indicate that S. glaucophyllus subsp. basinudus, S. glaucophyllus subsp. discoideus, and S. glaucophyllus subsp. toa are currently best considered as subspecies of S. matatini, new combinations for these three taxa are also provided. In addition, we lectotypify the name Senecio lautus var. montanus Cheeseman.

Loss of ecologically important genetic variation in late generation hybrids reveals links between adaptation and speciation.

Adaptation to contrasting environments occurs when advantageous alleles accumulate in each population, but it remains largely unknown whether these same advantageous alleles create genetic incompatibilities that can cause intrinsic reproductive isolation leading to speciation. Identifying alleles that underlie both adaptation and reproductive isolation is further complicated by factors such as dominance and genetic interactions among loci, which can affect both processes differently and obscure potential links between adaptation and speciation. Here, we use a combination of field and glasshouse experiments to explore the connection between adaptation and speciation while accounting for dominance and genetic interactions. We created a hybrid population with equal contributions from four contrasting ecotypes of Senecio lautus (Asteraceae), which produced hybrid genomes both before (F1 hybrid generation) and after (F4 hybrid generation) recombination among the parental ecotypes. In the glasshouse, plants in the second generation (F2 hybrid generation) showed reduced fitness as a loss of fertility. However, fertility was recovered in subsequent generations, suggesting that genetic variation underlying the fitness reduction was lost in subsequent generations. To quantify the effects of losing genetic variation at the F2 generation on the fitness of later generation hybrids, we used a reciprocal transplant to test for fitness differences between parental ecotypes, and F1 and F4 hybrids in all four parental habitats. Compared to the parental ecotypes and F1 hybrids, variance in F4 hybrid fitness was lower, and lowest in habitats that showed stronger native‐ecotype advantage, suggesting that stronger natural selection for the native ecotype reduced fitness variation in the F4 hybrids. Fitness trade‐offs that were present in the parental ecotypes and F1 hybrids were absent in the F4 hybrid. Together, these results suggest that the genetic variation lost after the F2 generation was likely associated with both adaptation and intrinsic reproductive isolation among ecotypes from contrasting habitats.

Natural selection drives leaf divergence in experimental populations of Senecio lautus under natural conditions.

Leaf morphology is highly variable both within and between plant species. This study employs a combination of common garden and reciprocal transplant experiments to determine whether differences in leaf shape between Senecio lautus ecotypes has evolved as an adaptive response to divergent ecological conditions.We created a synthetic population of hybrid genotypes to segregate morphological variation between three ecotypes and performed reciprocal transplants where this hybrid population was transplanted into the three adjacent native environments. We measured nine leaf morphology traits across the experimental and natural populations at these sites.We found significant divergence in multivariate leaf morphology toward the native character in each environment, suggesting environmental conditions at each site exert selective pressure that results in a phenotypic shift toward the local phenotype of the wild populations.These associations suggest that differences in leaf morphology between S. lautus ecotypes have arisen as a result of divergent selection on leaf shape or associated traits that confer an adaptive advantage in each environment, which has led to the formation of morphologically distinct ecotypes.

Genomic clustering of adaptive loci during parallel evolution of an Australian wildflower.

The build-up of the phenotypic differences that distinguish species has long intrigued biologists. These differences are often inherited as stable polymorphisms that allow the cosegregation of adaptive variation within species, and facilitate the differentiation of complex phenotypes between species. It has been suggested that the clustering of adaptive loci could facilitate this process, but evidence is still scarce. Here, we used QTL analysis to study the genetic basis of phenotypic differentiation between coastal populations of the Australian wildflower Senecio lautus. We found that a genomic region consistently governs variation in several of the traits that distinguish these contrasting forms. Additionally, some of the taxon-specific traits controlled by this QTL cluster have evolved repeatedly during the adaptation to the same habitats, suggesting that it could mediate divergence between locally adapted forms. This cluster contains footprints of divergent natural selection across the range of S.lautus, which suggests that it could have been instrumental for the rapid diversification of this species.

Diversification across a heterogeneous landscape

Adaptation to contrasting environments across a heterogeneous landscape favors the formation of ecotypes by promoting ecological divergence. Patterns of fitness variation in the field can show whether natural selection drives local adaptation and ecotype formation. However, to demonstrate a link between ecological divergence and speciation, local adaptation must have consequences for reproductive isolation. Using contrasting ecotypes of an Australian wildflower, Senecio lautus in common garden experiments, hybridization experiments, and reciprocal transplants, we assessed how the environment shapes patterns of adaptation and the consequences of adaptive divergence for reproductive isolation. Local adaptation was strong between ecotypes, but weaker between populations of the same ecotype. F1 hybrids exhibited heterosis, but crosses involving one native parent performed better than those with two foreign parents. In a common garden experiment, F2 hybrids exhibited reduced fitness compared to parentals and F1 hybrids, suggesting that few genetic incompatibilities have accumulated between populations adapted to contrasting environments. Our results show how ecological differences across the landscape have created complex patterns of local adaptation and reproductive isolation, suggesting that divergent natural selection has played a fundamental role in the early stages of species diversification.

Divergent natural selection drives the evolution of reproductive isolation in an Australian wildflower.

Ecological speciation occurs when reproductive isolation evolves between populations adapting to contrasting environments. A key prediction of this process is that the fitness of hybrids between divergent populations should be reduced in each parental environment as a function of the proportion of local genes they carry, a process resulting in ecologically dependent reproductive isolation (RI). To test this prediction, we use reciprocal transplant experiments between adjacent populations of an Australian wildflower, Senecio lautus, at two locations to distinguish between ecologically dependent and intrinsic genetic reproductive barriers. These barriers can be distinguished by observing the relative fitness of reciprocal backcross hybrids, as they differ in the contribution of genes from either parent while controlling for any intrinsic fitness effects of hybridization. We show ecologically dependent fitness effects in establishment and survival of backcrosses in one transplant experiment, and growth performance in the second transplant experiment. These results suggest natural selection can create strong reproductive barriers that maintain differentiation between populations with the potential to interbreed, and implies a significant role for ecology in the evolutionary divergence of S. lautus.

Senecio esperensis (Asteraceae: Senecioneae)—a new combination for the L'Esperance Rock groundsel, Kermadec Islands

A new combination, Senecio esperensis, is made for Senecio lautus subsp. esperensis, a narrow-range endemic confined to L'Esperance Rock, the southernmost of the Kermadec Islands group. Senecio esperensis is an ornithocoprophilous species wholly confined to the guano-enriched summit saddle of L'Esperance Rock. It is distinguished from S. lautus s.s. by its taller sparingly branched, strictly erect annual growth habit, succulent stems and foliage, and oblong-spathulate, irregularly serrate or lobulate, deeply toothed leaves. It has fewer involucral bracts, disc and ray-florets, shorter often twisted ray-florets, and smaller, sparingly hairy cypsela.

Tri-trophic interactions and the minimal effect of larval microsite and plant attributes on parasitism of Sphenella fascigera (Diptera: Tephritidae)

Parasitoid insects are important natural biocontrols of insect herbivores. How parasitoids choose particular hosts among many larvae on different plants is not fully understood. Our study system was the coastal plant Senecio lautus and its tephritid herbivore Sphenella fascigera parasitised by an undescribed parasitoid wasp Pteromalus sp. We located S. fascigera larvae in the field and reared pupae to investigate factors that might promote variation in parasitism: host larval location on the plant (stem gall versus flower head), plant density and quality, host larval density, and presence of other insect herbivores. The overall rate of parasitism of S. fascigera was 25%. There was no significant effect of larval location, or any other recorded variable, upon parasitism. One explanation could be the evolution of an efficient search strategy by Pteromalus sp. Future research on whether S. fascigera larvae on congeneric plants experience different rates of parasitism could contribute to understanding the larger web of interactions involving these species.

Strong extrinsic reproductive isolation between parapatric populations of an Australian groundsel

Speciation with gene flow, or the evolution of reproductive isolation between interbreeding populations, remains a controversial problem in evolution. This is because gene flow erodes the adaptive differences that selection creates between populations. Here, we use a combination of common garden experiments in the field and in the glasshouse to investigate what ecological and genetic mechanisms prevent gene flow and maintain morphological and genetic differentiation between coastal parapatric populations of the Australian groundsel Senecio lautus. We discovered that in each habitat extrinsic reproductive barriers prevented gene flow, whereas intrinsic barriers in F1 hybrids were weak. In the field, herbivores played a major role in preventing gene flow, but glasshouse experiments demonstrated that soil type also created variable selective pressures both locally and on a greater geographic scale. Our experimental results demonstrate that interfertile plant populations adapting to contrasting environments may diverge as a consequence of concurrent natural selection acting against migrants and hybrids through multiple mechanisms. These results provide novel insights into the consequences of local adaptation in the origin of strong barriers to gene flow in plants, and suggest that herbivory may play an important role in the early stages of plant speciation.

Genomic evidence for the parallel evolution of coastal forms in the Senecio lautus complex

Instances of parallel ecotypic divergence where adaptation to similar conditions repeatedly cause similar phenotypic changes in closely related organisms are useful for studying the role of ecological selection in speciation. Here we used a combination of traditional and next generation genotyping techniques to test for the parallel divergence of plants from the Senecio lautus complex, a phenotypically variable groundsel that has adapted to disparate environments in the South Pacific. Phylogenetic analysis of a broad selection of Senecio species showed that members of the S.lautus complex form a distinct lineage that has diversified recently in Australasia. An inspection of thousands of polymorphisms in the genome of 27 natural populations from the S.lautus complex in Australia revealed a signal of strong genetic structure independent of habitat and phenotype. Additionally, genetic differentiation between populations was correlated with the geographical distance separating them, and the genetic diversity of populations strongly depended on geographical location. Importantly, coastal forms appeared in several independent phylogenetic clades, a pattern that is consistent with the parallel evolution of these forms. Analyses of the patterns of genomic differentiation between populations further revealed that adjacent populations displayed greater genomic heterogeneity than allopatric populations and are differentiated according to variation in soil composition. These results are consistent with a process of parallel ecotypic divergence in face of gene flow.

CONVERGENCE AND DIVERGENCE DURING THE ADAPTATION TO SIMILAR ENVIRONMENTS BY AN AUSTRALIAN GROUNDSEL

Adaptation to replicate environments is often achieved through similar phenotypic solutions. Whether selection also produces convergent genomic changes in these situations remains largely unknown. The variable groundsel, Senecio lautus, is an excellent system to investigate the genetic underpinnings of convergent evolution, because morphologically similar forms of these plants have adapted to the same environments along the coast of Australia. We compared range-wide patterns of genomic divergence in natural populations of this plant and searched for regions putatively affected by natural selection. Our results indicate that environmental adaptation followed complex genetic trajectories, affecting multiple loci, implying both the parallel recruitment of the same alleles and the divergence of completely different genomic regions across geography. An analysis of the biological functions of candidate genes suggests that adaptation to coastal environments may have occurred through the recruitment of different genes participating in similar processes. The relatively low genetic convergence that characterizes the parallel evolution of S. lautus forms suggests that evolution is more constrained at higher levels of biological organization.

Plastic heteroblasty in beach groundsel (Senecio lautus)

Senecio lautus (Asteraceae) is a heteroblastic herb that produces entire juvenile leaves and lobed adult leaves. Juveniles commonly grow from rock fissures where they are shaded and sheltered from high winds, whereas adult plants are exposed to high light and wind as they outgrow these refuges. Because the duration each plant is sheltered varies with the depth of rock fissures, I hypothesised that heteroblasty in S. lautus varies plastically in response to environmental conditions. I tested this hypothesis in a glasshouse experiment, which exposed developing plants to different light and wind conditions. Plants grown in windy conditions showed similar ontogenetic changes in leaf morphology to plants growing in a control treatment. However, the juvenile‐adult transition was slowed in shaded conditions, indicating that heteroblasty in S. lautus varies plastically in response to light. Results were therefore consistent with the hypothesis that plastic heteroblasty is favoured in species that experience unpredictable environmental gradients during ontogeny.

Senecio repangae (Asteraceae): A new endemic species from the north‐eastern North Island, New Zealand

A new species Senecio repangae is described from the north‐eastern North Island, New Zealand, where it grows primarily on offshore islands. It has previously been included within the widespread New Zealand endemic Senecio lautus, from which it is distinguished by its much taller, sparingly branched habit, longer and narrower capitula, much shorter, widely and unevenly spaced, recurved, incised ray florets, and by its chromosome number (2n = 100). A new subspecies, S. repangae subsp. pokohinuensis, endemic to the remote Mokohinau Islands group is also recognised. This allopatric taxon is distinguished from S. repangae subsp. repangae by its sparsely hairy, glabrescent, glaucous, adaxial leaf surface, smaller involucral bracts, and evenly spaced, longer, ray florets which are never recurved. The distribution, ecology, cytology, reproductive biology, conservation, and relationships of both subspecies to other New Zealand Senecio species are assessed.

Pyrrolizidine alkaloidosis of cattle associated with Senecio lautus.

Serious incidents of pyrrolizidine alkaloidosis of cattle in 10 herds exposed to the Australian native plant, Senecio lautus (Asteraceae), were seen in central Queensland during 1988-1992. The deaths of 226 cattle were recorded. A mean of 8% of cattle died in affected groups (range 2 to 58%). Sickness and deaths usually occurred some months after access to S lautus. Typically, affected cattle lost body condition to the point of emaciation before dying and had persistent diarrhoea. Some animals developed abnormal behaviour and died after a shorter illness. Liver specimens from affected cattle in all herds contained lesions consistent with pyrrolizidine alkaloidosis. Thin layer chromatography of extracts of blood and liver samples from cattle from 5 herds detected pyrrolic metabolites. The identity of these was confirmed by mass spectroscopy on samples from one herd. Unseasonal autumn and winter rain after a dry summer appeared to favour growth of S lautus at the expense of other pasture species. A subsequent dry period promoted consumption of S lautus and was followed by a cluster of poisoning incidents.

Notes on theSenecio lautuscomplex in New Zealand

Abstract Two new species are accepted for the Senecio lautus complex in New Zealand. The first, S. carnosulus, previously treated as a variety or subspecies of S. lautus, is distinguished from S. lautus sens. strict. by being generally larger in all its parts (except the ligules which are shorter), by its more southerly distribution, and a chromosome number of 2n = 80 rather than 2n = 40. As defined here, S. carnosulus occurs in coastal areas from Banks Peninsula southwards to the islands of Foveaux Strait and on Stewart Island. The second species, S. marotiri, is newly described from offshore islands of northern New Zealand. It differs from S. lautus in the more erect habit, in leaf shape, bract and ligule length, and in having a chromosome number of 2n = 80, and from S. carnosulus in the erect habit, in leaf shape, and capitulum shape.

THE IDENTITY AND IMMATURE STAGES OF PATAGONIODES FARINARIA (TURNER) COMB. N. (LEPIDOPTERA: PYRALIDAE: PHYCITINAE) IN AUSTRALIA AND NEW ZEALAND

AbstractThe morphology of the immature stages and adult moth of the bivoltine blue stem borer, Patagoniodes farinaria (Turner) comb. n., is described and figured. Moths from eastern Australia and New Zealand appear to be conspecific and the species appears not to be introduced from Europe where related phycitines of similar biology occur. The blue stem borer exhibits features atypical of Homoeosoma Curtis, in which it was originally described, and generally conforms with Patagoniodes Roesler, to which it is here referred. Larvae develop in the stems of introduced ragwort, Senecio jacobaea L., and the native Senecio lautus Sol. ex Willd. (sens. lat.).