Exploring the ontogenetic scaling hypothesis during the diversification of pollination syndromes in Caiophora (Loasaceae, subfam. Loasoideae)

Floral Diversity and Pollination Syndromes in Agave subgenus Manfreda

The genus Agave is an ecological keystone of American deserts and both culturally and economically important in Mexico. Agave is a large genus of about 250 species. The radiation of Agave is marked by an initial adaptation to desert environments and then a secondary diversification of species associated with pollinator groups, such as hummingbirds and nocturnal moths. Phylogenetic analyses place Agave subgenus Manfreda, or the "herbaceous agaves," in a monophyletic clade that likely evolved in part as an adaptation to novel pollination vectors. Here, we present a morphological and observational study assessing the evolution of floral form in response to pollinator specialization within this understudied group. We found significant visitation by hummingbirds and nocturnal moths to several species within the Agave subgenus Manfreda. These observations also align with our morphological analyses of floral organs and support the evolution of distinct pollination syndromes. We found that not all floral morphology is consistent within a pollination syndrome, suggesting hidden diversity in the evolution of floral phenotypes in Agave. We also characterize the morphological variation between herbarium and live specimens, demonstrating that special consideration needs to be made when combining these types of data. This work identifies the potential for studying the functional evolution of diverse floral forms within Agave and demonstrates the need to further explore ecological and evolutionary relationships to understand pollinator influence on diversification in the genus.

Their young bite better: On- and off-host selection pressure as drivers for evolutionary-developmental modification in Rhipicephalus ticks

Distinct life stages may experience different selection pressures influencing phenotypic evolution. Morphological evolution is also constrained by early phenotypes, since early development forms the phenotypic basis of later development. This work investigates evolutionary-developmental modification in three life stages and both sexes of 24 Rhipicephalus species using phylogenetic comparative methods for geometric morphometrics of basis capituli (basal mouthpart structure used for host attachment), and scutum or conscutum areas (proxy for overall body size). Findings indicate species using large hosts at early life stages have distinct basis capituli shapes, correlated with host size, enabling attachment to the tough skins of large hosts. Host-truncate species (one- and two-host) generally retain these adaptive features into later life stages, suggesting neoteny is linked to the evolution of host truncation. In contrast, species using small hosts at early life stages have lost these features. Developmental trajectories differ significantly between host-use strategies (niches), and correlate with distinct clades. In two-host and three-host species using large hosts at early life stages, developmental change is heterotopically accelerated (greater cell mass development) before the first off-host period where selection probably favours large individuals able to better resist dehydration when questing (waiting) for less abundant, less active hosts. In other species, development is heterotopically reduced (neotenic), possibly because dehydration risk is bypassed by prolonged host attachment (one-host species - heterotopic neoteny), or is allometrically repatterned possibly by using highly abundant and active hosts (three-host species using small hosts at early life stages - allometric repatterning). These findings highlight complex trade-offs between on- and off-host factors of free-living ectoparasite ecology, which mediate responses to diverse selection pressures varied by life stage and host-use strategy. It is proposed that these trade-offs shape evolutionary-developmental morphology and diversity of Rhipicephalus ticks.

Methods for studying allometry in geometric morphometrics: a comparison of performance

Allometry has been the focus of growing interest in studies using geometric morphometric methods to address a wide range of research questions at the interface of ecology and evolution. This study uses computer simulations to compare four methods for estimating allometric vectors from landmark data: the multivariate regression of shape on a measure of size, the first principal component (PC1) of shape, the PC1 in conformation space, and a recently proposed method, the PC1 of Boas coordinates. Simulations with no residual variation around the allometric relationship showed that all four methods are logically consistent with one another, up to minor nonlinearities in the mapping between conformation space and shape tangent space. In simulations that included residual variation, either isotropic or with a pattern independent of allometry, regression of shape on size performed consistently better than the PC1 of shape. The PC1s of conformation and of Boas coordinates were very similar and very close to the simulated allometric vectors under all conditions. An extra series of simulations to elucidate the relation between conformation and Boas coordinates indicated that they are almost identical, with a marginal advantage for conformation. Empirical examples of ontogenetic allometry in rat skulls and rockfish body shape illustrate simple biological applications of the methods. The paper concludes with recommendations how these methods for estimating allometry can be used in studies of evolution and ecology.

Delayed differentiation of epidermal cells walls can underlie pedomorphosis in plants: the case of pedomorphic petals in the hummingbird-pollinated Caiophora hibiscifolia (Loasaceae, subfam. Loasoideae) species

Background

Understanding the relationship between macroevolutionary diversity and variation in organism development is an important goal of evolutionary biology. Variation in the morphology of several plant and animal lineages is attributed to pedomorphosis, a case of heterochrony, where an ancestral juvenile shape is retained in an adult descendant. Pedomorphosis facilitated morphological adaptation in different plant lineages, but its cellular and molecular basis needs further exploration. Plant development differs from animal development in that cells are enclosed by cell walls and do not migrate. Moreover, in many plant lineages, the differentiated epidermis of leaves, and leaf-derived structures, such as petals, limits organ growth. We, therefore, proposed that pedomorphosis in leaves, and in leaf-derived structures, results from delayed differentiation of epidermal cells with respect to reproductive maturity. This idea was explored for petal evolution, given the importance of corolla morphology for angiosperm reproductive success.

Results

By comparing cell morphology and transcriptional profiles between 5 mm flower buds and mature flowers of an entomophile and an ornitophile Loasoideae species (a lineage that experienced transitions from bee- to hummingbird-pollination), we show that evolution of pedomorphic petals of the ornithophile species likely involved delayed differentiation of epidermal cells with respect to flower maturity. We also found that developmental mechanisms other than pedomorphosis might have contributed to evolution of corolla morphology.

Conclusions

Our results highlight a need for considering alternatives to the flower-centric perspective when studying the origin of variation in flower morphology, as this can be generated by developmental processes that are also shared with leaves.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13227-021-00186-x.

The role of ontogenetic allometry and nonallometric flower shape variation in species-level adaptive diversification - Calceolaria polyrhiza (Calceolariaceae) as a case study

Organism shape changes predictably during ontogeny, resulting in specific patterns of ontogenetic allometry. In several plant and animal lineages, among-species variation in the shape of mature organisms mirrors variation along their growth trajectories. Hence, ontogenetic allometry is an important bias in evolution. This bias should be stronger at reduced evolutionary time scales, in which among-trait correlations had less time to evolve. Nevertheless, it was shown that adaptation of organism shape frequently involved departures from the ancestral ontogenetic allometry. Moreover, only a moderate fraction of shape variation is correlated with size during ontogeny. Hence, nonallometric variation in shape (NAVSh) is likely to contribute to adaptation, even at reduced evolutionary time scales. We explored the contributions of allometric variation in shape (AVSh), NAVSh, and size variation to adaptive evolution in the angiosperm species Calceolaria polyrhiza. This strongly relies on oil-collecting bees for pollination and experienced transitions in the size of pollinators during the last 2 Ma. Using geometric morphometrics, we described corolla morphology in several populations across its distribution range. Variation in corolla shape was decomposed into an allometric and a nonallometric component, and corolla size was estimated. We then looked for the correlation between these aspects of morphology and the pollinator. Our results suggest that adaptation to pollinators with different sizes relied on NAVSh, which resulted from shifts in the allometric slope and from shape changes that occurred early in flower development. We conclude that NAVSh can contribute to adaptation in flowering plants, even at the species-level.

PASOS: a method for the phylogenetic analysis of shape ontogenies

We present a novel phylogenetic approach to infer ancestral ontogenies of shape characters described as landmark configurations. The method is rooted in previously published theoretical developments to analyse landmark data in a phylogenetic context with parsimony as the optimality criterion, in this case using the minimization of differences in landmark position to define not only ancestral shapes but also the changes in developmental timing between ancestor-descendant shape ontogenies. Evolutionary changes along the tree represent changes in relative developmental timing between ontogenetic trajectories (possible heterochronic events) and changes in shape within each stage. The method requires the user to determine the shape of the specimens between two standard events, for instance birth and onset of sexual maturity. Once the ontogenetic trajectory is discretized into a series of consecutive stages, the method enables the user to identify changes in developmental timing associated with changes in the offset and/or onset of the shape ontogenetic trajectories. The method is implemented in a C language program called SPASOS. The analysis of two empirical examples (anurans and felids) using this novel method yielded results in agreement with previous hypotheses about shape evolution in these groups based on non-phylogenetic analyses.

Postmetamorphic ontogenetic allometry and the evolution of skull shape in Nest-building frogs Leptodactylus (Anura: Leptodactylidae)

Allometry constitutes an important source of morphological variation. However, its influence in head development in anurans has been poorly explored. By using geometric morphometrics followed by statistical and comparative methods we analyzed patterns of allometric change during cranial postmetamorphic ontogeny in species of Nest-building frogs Leptodactylus (Leptodactylidae). We found that the anuran skull is not a static structure, and allometry plays an important role in defining its shape in this group. Similar to other groups with biphasic life-cycle, and following a general trend in vertebrates, ontogenetic changes mostly involve rearrangement in rostral, otoccipital, and suspensorium regions. Ontogenetic transformations are paralleled by shape changes associated with evolutionary change in size, such that the skulls of species of different intrageneric groups are scaled to each other, and small and large species show patterns of paedomorphic/peramorphic features, respectively. Allometric trajectories producing those phenotypes are highly evolvable though, with shape change direction and magnitude varying widely among clades, and irrespective of changes in absolute body size. These results reinforce the importance of large-scale comparisons of growth patterns to understand the plasticity, evolution, and polarity of morphological changes in different clades.

Heterochronic Shifts Mediate Ecomorphological Convergence in Skull Shape of Microcephalic Sea Snakes

Morphological variation among the viviparous sea snakes (Hydrophiinae), a clade of fully aquatic elapid snakes, includes an extreme “microcephalic” ecomorph that has a very small head atop a narrow forebody, while the hind body is much thicker (up to three times the forebody girth). Previous research has demonstrated that this morphology has evolved at least nine times as a consequence of dietary specialization on burrowing eels, and has also examined morphological changes to the vertebral column underlying this body shape. The question addressed in this study is what happens to the skull during this extreme evolutionary change? Here we use X-ray micro-computed tomography and geometric morphometric methods to characterize cranial shape variation in 30 species of sea snakes. We investigate ontogenetic and evolutionary patterns of cranial shape diversity to understand whether cranial shape is predicted by dietary specialization, and examine whether cranial shape of microcephalic species may be a result of heterochronic processes. We show that the diminutive cranial size of microcephalic species has a convergent shape that is correlated with trophic specialization to burrowing prey. Furthermore, their cranial shape is predictable for their size and very similar to that of juvenile individuals of closely related but non-microcephalic sea snakes. Our findings suggest that heterochronic changes (resulting in pedomorphosis) have driven cranial shape convergence in response to dietary specializations in sea snakes.

Sexual size and shape dimorphism in two ground beetle taxa, Carabus (Procrustes) coriaceus cerisyi and C. (Morphocarabus) kollari praecellens (Coleoptera: Carabidae) - A geometric morphometric approach

We investigated morphometric variation in size and shape of the head, pronotum, and abdomen between the taxa and sexes of two ground beetles, Carabus coriaceus cerisyi Dejean, 1826 and C. kollari praecellens Palliardi, 1825. These two taxa differ in overall size, and both of them are characterized by significant sexual size dimorphism. In many taxa, allometry, the relationship between changes in shape and changes in size, can be a major component of intra- and interspecific variation in body shape. In the present study, we applied landmark-based geometric morphometrics to explore allometric and non-allometric components of shape variation between the taxa and more importantly between sexes within the taxa. We were able to show that the differences of shape between the taxa cannot be explained by allometric changes, as allometry explains only a small amount of total shape variation between the taxa, which was expected due to the fact that the taxa belong to separate subgenera. Between the sexes, on the other hand, allometry contributes largely to the variation, particularly in abdomen shape. However, the differences of head and pronotum shape between the sexes cannot be entirely explained in terms of allometric scaling. Our results indicate that allometry contributes largely to differences of body shape between the sexes, while differences between the taxa are influenced by other factors and processes.

Changes in ontogenetic patterns facilitate diversification in skull shape of Australian agamid lizards

BACKGROUND

Morphological diversity among closely related animals can be the result of differing growth patterns. The Australian radiation of agamid lizards (Amphibolurinae) exhibits great ecological and morphological diversity, which they have achieved on a continent-wide scale, in a relatively short period of time (30 million years). Amphibolurines therefore make an ideal study group for examining ontogenetic allometry. We used two-dimensional landmark-based geometric morphometric methods to characterise the postnatal growth patterns in cranial shape of 18 species of amphibolurine lizards and investigate the associations between cranial morphology, and life habit and phylogeny.

RESULTS

For most amphibolurine species, juveniles share a similar cranial phenotype, but by adulthood crania are more disparate in shape and occupy different sub-spaces of the total shape space. To achieve this disparity, crania do not follow a common post-natal growth pattern; there are differences among species in both the direction and magnitude of change in morphospace. We found that these growth patterns among the amphibolurines are significantly associated with ecological life habits. The clade Ctenophorus includes species that undergo small magnitudes of shape change during growth. They have dorsoventrally deep, blunt-snouted skulls (associated with terrestrial lifestyles), and also dorsoventrally shallow skulls (associated with saxicolous lifestyles). The sister clade to Ctenophorus, which includes the bearded dragon (Pogona), frill-neck lizard (Chlamydosaurus), and long-nosed dragon (Gowidon), exhibit broad and robust post-orbital regions and differing snout lengths (mainly associated with scansorial lifestyles).

CONCLUSIONS

Australian agamids show great variability in the timing of development and divergence of growth trajectories which results in a diversity of adult cranial shapes. Phylogenetic signal in cranial morphology appears to be largely overwritten by signals that reflect life habit. This knowledge about growth patterns and skull shape diversity in agamid lizards will be valuable for placing phylogenetic, functional and ecological studies in a morphological context.

Contrasting lengths of Pelargonium floral nectar tubes result from late differences in rate and duration of growth

BACKGROUND AND AIMS

Much of morphological evolution in flowers has arisen from pollinator-mediated selection, often manifest as a match between the length of the pollinator's proboscis and the depth of tubular corollas or spurs. We investigate development, growth and homology of the unique nectar tube of Pelargonium, frequently described as 'a spur adnate to the pedicel'.

METHODS

We focused on two species. The nectar tube of P. ionidiflorum is three times longer than that of P. odoratissimum. Light and scanning electron microscopy were carried out, and daily growth measurements were used to compare nectar tube development and vascular patterns.

KEY RESULTS

Nectar tubes in both species are initiated centripetally to the dorsal sepal in a space created by lateral displacement of two antepetalous stamens. The cavity deepens through subsequent intercalary growth of the receptacle that proceeds at the same rate in both species until tubes reach approx. 10 mm in length. Differences in final nectar tube lengths arise via an increase in the rate and duration of growth of the receptacle that begins just before anthesis (floral opening) and continues for several days past anthesis in P. ionidiflorum but does not occur in P. odoratissimum. Epidermal cells of the dorsal surface of the nectar tube in P. ionidiflorum are approx. 1.6 times longer than those in P. odoratissimum. Histological sections show no evidence that the nectar tube is a spur that became evolutionarily fused to the pedicel.

CONCLUSIONS

Nectar tubes in Pelargonium are localized cavities that form in the receptacle via intercalary growth. Differences in the rate and duration of growth just prior to and following anthesis underlie differences in final tube lengths. Because differences in cell lengths do not fully account for differences in nectar tube lengths, evolutionary diversification must involve changes in both cell cycle and cell expansion.

The evolution of floral ontogenetic allometry in the Andean genus Caiophora (Loasaceae, subfam. Loasoideae)

The astounding variety of angiosperm flower morphologies has evolved in response to many selective forces. Flower development is highly coordinated and involves developmental associations between size and shape, ontogenetic allometry, which in turn affect the morphology of mature flowers. Although ontogenetic allometries can act as a developmental constraint and may influence adaptive evolution, allometries can evolve themselves and may change rapidly in response to selection. We explored the evolution of ontogenetic allometry in the flowers of 11 species of Loasoideae. Seven species belong to Caiophora, which radiated recently in the central Andes, and contains species that are pollinated by bees, hummingbirds, and small rodents. According to a previous study, the diversification of Caiophora involved departures from simple allometric scaling, but the changes to allometry that enabled flower diversification have not been explored yet. We characterized the ontogenetic allometry of each species with the methods of geometric morphometrics. We studied the evolution of allometries by constructing allometric spaces, in which the allometry of each species is represented by a point and the arrangement of points indicates the relations among allometric trajectories. To examine the history of changes of ontogenetic allometries, we projected the phylogeny into the allometric spaces. Inspection of allometric spaces suggests that ontogenetic variation is limited to a few dominant features. The allometries of the two main functional flower parts under study differ in their evolutionary labilities, and patterns of variation reflect pollination systems, differences in structural organization, and abiotic environmental factors.

A potential pitfall in studies of biological shape: Does size matter?

The number of published studies using geometric morphometrics (GM) for analysing biological shape has increased steadily since the beginning of the 1990s, covering multiple research areas such as ecology, evolution, development, taxonomy and palaeontology. Unfortunately, we have observed that many published studies using GM do not evaluate the potential allometric effects of size on shape, which normally require consideration or assessment. This might lead to misinterpretations and flawed conclusions in certain cases, especially when size effects explain a large part of the shape variation. We assessed, for the first time and in a systematic manner, how often published studies that have applied GM consider the potential effects of allometry on shape. We reviewed the 300 most recent published papers that used GM for studying biological shape. We also estimated how much of the shape variation was explained by allometric effects in the reviewed papers. More than one-third (38%) of the reviewed studies did not consider the allometric component of shape variation. In studies where the allometric component was taken into account, it was significant in 88% of the cases, explaining up to 87.3% of total shape variation. We believe that one reason that may cause the observed results is a misunderstanding of the process that superimposes landmark configurations, i.e. the Generalized Procrustes Analysis, which removes isometric effects of size on shape, but not allometric effects. Allometry can be a crucial component of shape variation. We urge authors to address, and report, size effects in studies of biological shape. However, we do not propose to always remove size effects, but rather to evaluate the research question with and without the allometric component of shape variation. This approach can certainly provide a thorough understanding of how much size contributes to the observed shaped variation.

Getting a head in hard soils: Convergent skull evolution and divergent allometric patterns explain shape variation in a highly diverse genus of pocket gophers (Thomomys)

Background

High morphological diversity can occur in closely related animals when selection favors morphologies that are subject to intrinsic biological constraints. A good example is subterranean rodents of the genus Thomomys, one of the most taxonomically and morphologically diverse mammalian genera. Highly procumbent, tooth-digging rodent skull shapes are often geometric consequences of increased body size. Indeed, larger-bodied Thomomys species tend to inhabit harder soils. We used geometric morphometric analyses to investigate the interplay between soil hardness (the main extrinsic selection pressure on fossorial mammals) and allometry (i.e. shape change due to size change; generally considered the main intrinsic factor) on crania and humeri in this fast-evolving mammalian clade.

Results

Larger Thomomys species/subspecies tend to have more procumbent cranial shapes with some exceptions, including a small-bodied species inhabiting hard soils. Counter to earlier suggestions, cranial shape within Thomomys does not follow a genus-wide allometric pattern as even regional subpopulations differ in allometric slopes. In contrast, humeral shape varies less with body size and with soil hardness. Soft-soil taxa have larger humeral muscle attachment sites but retain an orthodont (non-procumbent) cranial morphology. In intermediate soils, two pairs of sister taxa diverge through differential modifications on either the humerus or the cranium. In the hardest soils, both humeral and cranial morphology are derived through large muscle attachment sites and a high degree of procumbency.

Conclusions

Our results show that conflict between morphological function and intrinsic allometric patterning can quickly and differentially alter the rodent skeleton, especially the skull. In addition, we found a new case of convergent evolution of incisor procumbency among large-, medium-, and small-sized species inhabiting hard soils. This occurs through different combinations of allometric and non-allometric changes, contributing to shape diversity within the genus. The strong influence of allometry on cranial shape appears to confirm suggestions that developmental change underlies mammalian cranial shape divergences, but this requires confirmation from ontogenetic studies. Our findings illustrate how a variety of intrinsic processes, resulting in species-level convergence, could sustain a genus-level range across a variety of extrinsic environments. This might represent a mechanism for observations of genus-level niche conservation despite species extinctions in mammals.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0782-1) contains supplementary material, which is available to authorized users.

Size, shape, and form: concepts of allometry in geometric morphometrics

Allometry refers to the size-related changes of morphological traits and remains an essential concept for the study of evolution and development. This review is the first systematic comparison of allometric methods in the context of geometric morphometrics that considers the structure of morphological spaces and their implications for characterizing allometry and performing size correction. The distinction of two main schools of thought is useful for understanding the differences and relationships between alternative methods for studying allometry. The Gould-Mosimann school defines allometry as the covariation of shape with size. This concept of allometry is implemented in geometric morphometrics through the multivariate regression of shape variables on a measure of size. In the Huxley-Jolicoeur school, allometry is the covariation among morphological features that all contain size information. In this framework, allometric trajectories are characterized by the first principal component, which is a line of best fit to the data points. In geometric morphometrics, this concept is implemented in analyses using either Procrustes form space or conformation space (the latter also known as size-and-shape space). Whereas these spaces differ substantially in their global structure, there are also close connections in their localized geometry. For the model of small isotropic variation of landmark positions, they are equivalent up to scaling. The methods differ in their emphasis and thus provide investigators with flexible tools to address specific questions concerning evolution and development, but all frameworks are logically compatible with each other and therefore unlikely to yield contradictory results.

Size, shape, and form: concepts of allometry in geometric morphometrics

Allometry refers to the size-related changes of morphological traits and remains an essential concept for the study of evolution and development. This review is the first systematic comparison of allometric methods in the context of geometric morphometrics that considers the structure of morphological spaces and their implications for characterizing allometry and performing size correction. The distinction of two main schools of thought is useful for understanding the differences and relationships between alternative methods for studying allometry. The Gould–Mosimann school defines allometry as the covariation of shape with size. This concept of allometry is implemented in geometric morphometrics through the multivariate regression of shape variables on a measure of size. In the Huxley–Jolicoeur school, allometry is the covariation among morphological features that all contain size information. In this framework, allometric trajectories are characterized by the first principal component, which is a line of best fit to the data points. In geometric morphometrics, this concept is implemented in analyses using either Procrustes form space or conformation space (the latter also known as size-and-shape space). Whereas these spaces differ substantially in their global structure, there are also close connections in their localized geometry. For the model of small isotropic variation of landmark positions, they are equivalent up to scaling. The methods differ in their emphasis and thus provide investigators with flexible tools to address specific questions concerning evolution and development, but all frameworks are logically compatible with each other and therefore unlikely to yield contradictory results.

Robust views on plasticity and biodiversity

Background How the diversity of life on our planet originated is not completely understood and many questions are still open. Especially, the role of developmental robustness in evolution is an often neglected topic.

Scope Considering diverse groups of plants and animals, and employing different concepts and approaches, the authors of articles in this Special Issue try to understand better the impact of developmental robustness, phenotypic plasticity and variance on species diversity, evolution and morphological disparity.

Conclusions Several lines of theoretical considerations as well as case studies show that developmental robustness supports rather than prevents the evolution of species diversity, at least under certain circumstances. Among the possible mechanisms is the scenario that developmental robustness facilitates the synorganization of body parts, which may enable the origin of complex novelties; this then may set the ground for species radiation.