Source Of Morphological Variation Research Articles

Morphometric analysis of the Late Cretaceous Placenticeras of Alabama, USA: sexual dimorphism, allometry, and implications for taxonomy

AbstractA traditional typological approach to taxonomy often does not adequately account for intraspecific variation and can result in taxonomic oversplitting. For many groups, including ammonoids of the Placenticeras genus, intraspecific variation documented in recent studies (e.g., ontogenetic changes, sexual dimorphism, polymorphism) challenges the historic proliferation of species names. Here, we used a population approach to taxonomy and quantitatively evaluated morphometric variation in a sample of Late Cretaceous (Santonian–Campanian) Placenticeras from Alabama and adjacent counties.We used linear mixed models (LMMs) to characterize how morphological variables scale with conch size across the sample, exploiting mixed longitudinal data to evaluate individual variation in growth and inform interpretations of multivariate analyses. Extended LMMs incorporating geological formation evaluated morphological changes through time. Principal component analysis and clustering analysis were then used to evaluate the number of distinct clusters that emerged in multivariate morphospace independent of previous taxon name assignments.Discontinuous scaling relationships and distinct clusters in multivariate space suggest sexual dimorphism characterized by differences in adult size and, secondarily, shape. Previous Stantonoceras and Placenticeras assignments broadly overlap in our morphospace, failing to justify this historic distinction (as sexual dimorphs or as genera or subgenera). Placenticeras conch morphology and ornament placement changed through time, suggesting a potential utility for coarse (stage-level) biostratigraphy. However, temporal changes were not associated with distinct clusters in morphospace, and our data fail to support the plethora of reported species names. As few as one or two (successive) species may be present in our sample (representing 130 years of collection effort). In addition to highlighting the need for a significant taxonomic revision of the Placenticeras genus, this study demonstrates the utility of LMMs for distinguishing between different sources of morphological variation, improving interpretations of morphospace under a population approach to taxonomy, and maximizing the amount of ontogenetic information that can be obtained nondestructively.

Organic-walled microfossils from the lower Cambrian of North Greenland: a reappraisal of diversity

The early Cambrian Buen Formation (North Greenland) hosts an exceptionally rich fossil biota that has contributed significantly to our knowledge of early metazoans, yet the fossil remains of primary producers from this deposit have received less attention. Here we examine the palynological component of the Buen Formation, with a focus on acritarchs and filamentous microfossils. Our analysis revealed the presence of 49 form taxa, 15 of which are described for the first time in the Buen Formation. These include large elements of presumably benthic origin, together with cyst-like acritarchs. Comasphaeridium longispinosum Vidal 1993 is renamed Comasphaeridium? brillesensis nom. nov., and Comasphaeridium densispinosum Vidal 1993 is reassigned to a new genus, Pearisphaeridium, becoming Pearisphaeridium densispinosum comb. nov. The diagnoses of Pearisphaeridium densispinosum (Vidal 1993) comb. nov. and Skiagia pura Moczydłowska 1988 are emended. Further, careful analysis of disparity in the recovered assemblage has revealed the presence of numerous transitional morphologies among the recorded acritarch form taxa. Though some of these transitional forms likely represent biologically meaningful entities (e.g. life cycle stages, ecophenotypes), others appear to have been artificially generated by taphonomic processes. Accounting for taphonomic factors and other sources of morphological variation has curtailed diversity down to 30 acritarch morphotypes, ten of which represent distinct abundance peaks broadly corresponding to acritarch genera. This analysis illustrates how population-based studies of early Cambrian acritarchs can help to discern the different factors that impinge on acritarch morphology, detect instances of taxonomic inflation, and refine our measures of diversity at the base of early Palaeozoic food webs.

Cis-regulatory landscapes in the evolution and development of the mammalian skull.

Extensive morphological variation found in mammals reflects the wide spectrum of their ecological adaptations. The highest morphological diversity is present in the craniofacial region, where geometry is mainly dictated by the bony skull. Mammalian craniofacial development represents complex multistep processes governed by numerous conserved genes that require precise spatio-temporal control. A central question in contemporary evolutionary biology is how a defined set of conserved genes can orchestrate formation of fundamentally different structures, and therefore how morphological variability arises. In principle, differential gene expression patterns during development are the source of morphological variation. With the emergence of multicellular organisms, precise regulation of gene expression in time and space is attributed to cis-regulatory elements. These elements contribute to higher-order chromatin structure and together with trans-acting factors control transcriptional landscapes that underlie intricate morphogenetic processes. Consequently, divergence in cis-regulation is believed to rewire existing gene regulatory networks and form the core of morphological evolution. This review outlines the fundamental principles of the genetic code and genomic regulation interplay during development. Recent work that deepened our comprehension of cis-regulatory element origin, divergence and function is presented here to illustrate the state-of-the-art research that uncovered the principles of morphological novelty. This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Fluctuating Asymmetry in the head shape and parotid gland secretion in introduced cane toads (Rhinella marina) from established populations in Caraga State University, Butuan City, Agusan del Norte, Philippines

The study was supervised to determine and assess the ecological health of Caraga State University in Ampayon, Butuan City by looking into the level of fluctuating asymmetry in populations and affected landmarks of Rhinella marina. Various studies have shown that fluctuating asymmetry can be used to indirectly assess water quality as well as the overall health of the ecosystem. In this study, R. marina was utilized because of its abundance in Caraga State University. Hence, with the presence and use of the thin-plate spline (TPS) series, landmark analysis was obtained and subjected to Symmetry and Asymmetry in Geometric Data (SAGE) software. Wherein, the findings in Procrustes ANOVA showed that although individual symmetry delineate no significant difference, Sides (Directional Asymmetry) and Interaction (Fluctuating asymmetry) showed a highly significant difference (P = 0.0000***). Furthermore, the results of Principal Component Scores display a high percentage Fluctuating Asymmetry of pooled (79.964%). Thus, in the results, PC 1 (40.589%), PC2 (18.8634%), PC3 (12.7566%), and PC4 (7.755%), the interaction was explicitly found to have significant variations affecting the R. marina parotid gland, landmarks of the loreal crest, and supra-tympanic edge of the cane toad. A significant proportion of the variation in head shape can be explained by environmental variables, suggesting that conditions of the physical environment should also be considered as a source of morphological variation. To conclude, this study authenticated the use of fluctuating asymmetry in determining and assessing the status of the ecological health of the study area confirming that indeed results of high FA equates for the incidence of cane toads in the area.

Drosophila Wing Integration and Modularity: A Multi-Level Approach to Understand the History of Morphological Structures.

Simple SummaryThe diverse components of any morphological structure are integrated with respect to each other since they have developed, functioned, and evolved together, a phenomenon known as integration. However, this integration is not absolute but organized in units (i.e., modules) that are relatively independent while participating to generate a structure that acts as a functional whole. Even though most of the studies on modularity and integration have focused on variation among individuals within populations, there are more levels of variation that exhibit modularity and integration, deriving from distinct sources such as genetic variation, phenotypic plasticity, fluctuating asymmetry, evolutionary change, among others. Consequently, the present study focused on analysing the integration and modularity of the wing shape of some of the best-known model organisms, i.e., the genus Drosophila, at the static, developmental, and evolutionary levels to acquire a better insight about how modularity and integration act at different analytical levels. The strong integration and overall similarities observed in the variation pattern at multiple levels suggest a shared mechanism underlying the observed variation in Drosophila’s wing shape and added a new piece of evidence of stasis in the evolutionary history of Drosophila wing.Static, developmental, and evolutionary variation are different sources of morphological variation which can be quantified using morphometrics tools. In the present study we have carried out a comparative multiple level study of integration (i.e., static, developmental, and evolutionary) to acquire insight about the relationships that exist between different integration levels, as well as to better understand their involvement in the evolutionary processes related to the diversification of Drosophila’s wing shape. This approach was applied to analyse wing evolution in 59 species across the whole genus in a large dataset (~10,000 wings were studied). Static integration was analysed using principal component analysis, thus providing an integration measurement for overall wing shape. Developmental integration was studied between wing parts by using a partial least squares method between the anterior and posterior compartments of the wing. Evolutionary integration was analysed using independent contrasts. The present results show that all Drosophila species exhibit strong morphological integration at different levels. The strong integration and overall similarities observed at multiple integration levels suggest a shared mechanism underlying this variation, which could result as consequence of genetic drift acting on the wing shape of Drosophila.

Pond characteristics influence the intraspecific variation in the morphometry of the tadpoles of two species of Dendropsophus (Anura: Hylidae) from the Cerrado savanna of northeastern Brazil.

Spatial variation in the environment may be an important source of morphological variation in many organisms. Tadpoles are valuable model organisms for studies of morphological variation, and in particular, the evaluation of the relationship between environmental and morphological variables. In heterogeneous environments, such as the temporary ponds found in the Cerrado savanna of central Brazil, understanding how environmental variables influence the morphological variation found in tadpole populations can provide important insights into this phenomenon. The present study thus aimed to (i) describe the morphometric variation in tadpoles found in different populations of Dendropsophus nanus and Dendropsophus minutus in the Cerrado of southern Maranhão state (Brazil), and (ii) relate this variation in tadpole morphology to the characteristics of the local ponds. Tadpoles were collected from 11 pounds in southern Maranhão, and the morphological space among the different populations was compared using NPMANOVA, separately for each species. The degree of association between the environmental and morphological matrices was then tested using a Stepwise Multiple Linear Regression. The morphological (tadpole morphometry) and environmental (pond characteristics) data matrices were obtained by ordination techniques. Considerable morphometric differences were found among populations in both species. In D. nanus, the morphometric variation was correlated with that of the substrate, whereas in D. minutus, morphometry was associated with the vegetation found in the pond. Overall, then, the study demonstrated that distinct environmental variables influenced significantly the morphometry of the tadpoles of each frog species.

The peripheral nerves of Lepidobatrachus tadpoles (Anura, Ceratophryidae)

The peripheral nervous system of anuran larvae has traditionally been assumed to be largely invariant. Here, we describe the organization of cranial, spinal, and lateral line nerves at different larval stages of Lepidobatrachus spp. based on whole mounts. This is the first detailed description of cranial, spinal, and lateral lines innervation at premetamorphic stages of anuran larvae with notes on temporal variation. We distinguish three sources of morphological variation with respect to other anuran larvae: (a) the loss or reduction of some exclusively larval elements (i.e., the absence of the middle lateral line nerve); (b) spatial changes in the lateral line system (i.e., the supralabial arrangement of component of the anteroventral lateral line nerve); and (c) temporal changes in the disappearance of most of the lateral line system and in the premetamorphic repatterning of the spatial relationships of mandibularis ramus of the trigeminal (V) and hyomandibularis ramus of facial (VII). The innervation of limbs is achieved during late larval stages. Furthermore, comparisons among selected anurans reveal differences in tadpole brain morphology. The spatial and temporal variation found in the peripheral nerves of Lepidobatrachus larvae testifies to previously unappreciated variation in anuran larval morphology.

Geographical variation in head shape of a Neotropical group of toads: the role of physical environment and relatedness

In this study we review the morphological variation within the Rhinella crucifer species group using geometric morphometrics. We sampled 270 specimens from 78 localities comprising all genetic units delimited for the group. We placed 12 landmarks and 89 semi-landmarks defining morphological structures of the anterior region of the body (head and parotoid glands) on standardized photographs of dorsal aspects of specimens. We checked for the existence of size-free morphological variation using exploratory multivariate analyses and tested for differences among categories (genetic units) using canonical variate analyses. We investigated the effects of relatedness by conducting canonical analyses hierarchically, and tested for phylogenetic signal using reconstruction of morphologies on a tree derived from mitochondrial data. We then corrected for relatedness using phylogenetic principal component analysis, and tested for the influence of the physical environment (temperature, humidity and altitude) with a partial Mantel test of matrix correlation. Our results revealed that there is size-free shape variation in the group. Shape changes are related to specific structures in the head, with landmarks and semi-landmarks highlighting changes in a complementary way. We were able to statistically detect the effect of phylogenetic distance with landmarks when considering the closest genetic units as a single category. A significant proportion of the variation in head shape can be explained by environmental variables, suggesting that conditions of the physical environment should also be considered as a source of morphological variation.

Morphological mutations: lessons from the cockscomb.

Morphological mutations: lessons from the cockscomb.

Position of the Steinheim interglacial sequence within the marine oxygen isotope record based on mammal biostratigraphy

Since the recovery, in 1933, of a Homo heidelbergensis skull from the interglacial Antiquus-Schotter in sediments of the River Murr, the site of Steinheim (Germany) has been regarded as an iconic hominin locality for the Middle Pleistocene of Europe. Based on the morphology of the specimen, stratigraphical considerations and the characteristics of the associated faunal assemblage, the Steinheim skull has generally been assigned to the Holsteinian Interglacial. Over the last decades, developments in the knowledge of the complexity of the Pleistocene glacial–interglacial cycles have rendered this date increasingly problematic.Analyses of caballoid horse remains from the site using log ratio diagrams and principal components analysis reveal striking morphological differences with German horse remains from sites attributed to the Holsteinian or marine isotope stages (MIS) 11 and 9. The morphology of the Steinheim horses is similar to that of horse specimens from MIS 9 sites in southern France, and to a lesser degree there are similarities with MIS 9 horses from northern France and British MIS 11 and 9 horse fossils. This indicates that during the Anglian/Elsterian glacial, European horse populations were split into two lineages. During MIS 11, the western lineage is present in the British Isles and at Steinheim, whereas the German Holsteinian samples belong to the eastern lineage. A date in different substages of MIS 11 is a further possible source of morphological variation between these assemblages. By MIS 9, a degree of admixture had taken place between the eastern and western lineages.

Genetic basis of sexual dimorphism in the threespine stickleback Gasterosteus aculeatus

Sexual dimorphism (SD) in morphological, behavioural and physiological features is common, but the genetics of SD in the wild has seldom been studied in detail. We investigated the genetic basis of SD in morphological traits of threespine stickleback (Gasterosteus aculeatus) by conducting a large breeding experiment with fish from an ancestral marine population that acts as a source of morphological variation. We also examined the patterns of SD in a set of 38 wild populations from different habitats to investigate the relationship between the genetic architecture of SD of the marine ancestral population in relation to variation within and among natural populations. The results show that genetic architecture in terms of heritabilities, additive genetic variances and covariances (as well as correlations) is very similar in the two sexes in spite of the fact that many of the traits express significant SD. Furthermore, population differences in threespine stickleback body shape and armour SD appear to have evolved despite constraints imposed by genetic architecture. This implies that constraints for the evolution of SD imposed by strong genetic correlations are not as severe and absolute as commonly thought.

Morphological and molecular variation within an ocean basin in wedge-tailed shearwaters (Puffinus pacificus)

Micro-evolutionary processes that underpin genetic and morphological variation in highly mobile pelagic vertebrates are virtually unknown. Previous findings preferentially invoke vicariant isolation due to large-scale physical barriers such as continental landmasses, followed by genetic drift. However increasingly, evidence for divergence by non-random processes (e.g. selection, plasticity) is being presented. Wedge-tailed shearwaters are wide-ranging seabirds with breeding colonies located such that they experience a variety of environmental pressures and conditions. Previous work on this species has provided evidence of inter-colony divergence of adult morphology and foraging modes, as well as chick developmental patterns, suggesting that reinforcement among colonies is possible. In order to evaluate the micro-evolutionary processes driving this observed variation, our study compared patterns of gene flow with morphological and environmental variation among four colonies of wedge-tailed shearwater breeding within the Indo-Pacific Ocean basin. Estimates of gene flow differed according to the genetic marker used; most likely, this is a function of different mutation rates. Nuclear introns suggest that gene flow among wedge-tailed shearwater breeding colonies within the Indo-Pacific Ocean basin is substantial, however microsatellite markers imply that gene flow is reduced. In general, levels of genetic divergence were relatively low and did not correlate with geographic distance, morphological distance or environmental differences (sea-surface temperature and chlorophyll a concentration) among colonies. We suggest that genetic drift alone is unlikely to be the major source of morphological variation seen in this species. Instead, we propose that non-random processes (selection, plasticity) underpin morphological diversity seen in this and possibly other seabird species.

APPEARANCE OF MORPHOLOGICAL NOVELTY IN A HYBRID ZONE BETWEEN TWO SPECIES OF LAND SNAIL

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.

APPEARANCE OF MORPHOLOGICAL NOVELTY IN A HYBRID ZONE BETWEEN TWO SPECIES OF LAND SNAIL

On the small oceanic island of Chichijima, two endemic species of land snails, Mandarina mandarina and M. chichijimana, have discrete distributions separated by a hybrid zone. This study investigates the potential of hybridization as a source of morphological novelty in these snails. Mandarina mandarina possesses a shell with a higher whorl expansion rate and a smaller protoconch than M. chichijimana, relative to shell size. The number of whorls and shell size of M. mandarina do not differ from those of M. chichijimana, because the effect of higher expansion rate on number of whorls and size of the former is compensated for by its smaller protoconch. The whorl expansion rate and protoconch diameter of the individuals from the hybrid populations are intermediate or typical of either of the two species, and their average values show clinal changes along the hybrid zone. However, the hybrid populations include exceptionally high shells with many whorls and flat shells with few whorls, which are never found in the pure populations of either species. In addition, gradual increases in variance in shell height and number of whorls were found from the edges to the center of the hybrid zone. A combination of low expansion rate (typical of M. chichijimana) and a small protoconch (typical of M. mandarina) produces a shell with an extremely large number of whorls because of the geometry of shell coiling. However, the combination of high expansion rate and a large protoconch produces a shell with an extremely small number of whorls. Because of the correlation between the number of whorls and shell height, shells with an exceptional number of whorls possess an extraordinarily high or flat spire. Hybrids can inherit a mosaic of characters that, as they play out during growth, lead to novel adult morphologies. These findings emphasize the importance of hybridization as a source of morphological variation and evolutionary novelty in land snails.

Evolution and Phylogeny of Gonad Morphology in Bony Fishes

Gonad morphology at the gross anatomical or histological levels has long been studied by fisheries biologists to identify annual reproductive cycles and length of breeding season, among other goals. Comparative surveys across vertebrate taxa have not been detailed enough, however, to describe fully the differences and similarities among gonads of bony fishes and other vertebrates, and to use gonad morphology in phylogenetic systematic analyses. An emerging constant among vertebrates is the presence of a germinal epithelium composed of somatic and germ cells in both males and females. In females, the germinal epithelium lines the ovarian lamellae. In males, arrangement of the germinal epithelium into compartments varies among osteichthyans: basal taxa have an anastomosing tubular testis, whereas derived taxa have a lobular testis. The lobular testis is proposed as a synapomorphy of the Neoteleostei. The annual reproductive cycle is hypothesized to be the source of morphological variation among testis types. Elongation of germinal compartments during early maturation may result in a transition from anastomosing tubular to lobular testes. In all male atherinomorphs surveyed, spermatogonia are restricted to the distal termini of lobules rather than being distributed along the lobule; there is an epithelioid arrangement of Sertoli and germ cells rather than a germinal epithelium. Arrest of the maturation-regression phases is hypothesized to lead to formation of the atherinomorph testis. Atherinomorphs also have a distinctive egg with fluid, rather than granular, yolk. Variation among germinal epithelia is interpreted in a developing phylogenetic framework to understand evolution of gonad morphology and to propose gonad characters for phylogenetic analyses.

Nonheterochronic developmental changes underlie morphological heterochrony in the evolution of the Ardeidae

Abstract Evolutionary changes in developmental timing and rates (heterochrony) are a source of morphological variation. Here we explore a central issue in heterochronic analysis: are the alterations in developmental timing and rates the only factor underlying morphological heterochrony? Tarsometatarsal growth through endochondral ossification in Ardeidae evolution has been taken as a case study. Evolutionary changes in bone growth rate (morphological heterochrony) might be either (a) the result of alterations in the mitotic frequency of epiphyseal chondrocytes (process-heterochrony hypothesis), or (b) the outcome of alterations in the number of proliferating cells or in the size of hypertrophic chondrocytes (structural hypothesis). No correlation was found between tarsometatarsal growth rates and the frequency of cell division. However, bone growth rates were significantly correlated with the number of proliferating cells. These results support the structural hypothesis: morphological acceleration and deceleration are the outcome of evolutionary changes in one structural variable, the number of proliferating cells.

Novel colour polymorphisms in a hybrid zone of Mandarina (Gastropoda: Pulmonata)

A hybrid zone of the land snails Mandarina mandarina and Mandarina chichijimana in the oceanic Bonin Islands was studied morphologically and genetically to show the potential of hybridization as a source of morphological novelties. These species are clearly distinguishable on the basis of allozymes, colour polymorphisms, shell form and genital morphology, but exhibit a hybrid zone from the northeast to southwest of Chichijima island. There is a cline in the frequency of the colour patterns characteristic of each of the species, and specimens with intermediate colour pattern on the shells appear in the hybrid zone. Not only specimens with colour patterns that are overlapping of the patterns of the two species, but also specimens with unique colour patterns appear in the hybrid populations. These unique colour patterns are not found in the pure populations of both species or other Mandarina species in Chichijima Islands, and it is suggested that these are produced by the hybridization. Because of the appearance of many types of unique colour patterns, variability of the colour polymorphism in the hybrid populations are remarkably higher than that in the pure populations. This result suggests that the novel morphology is produced by the hybridization between species with distinctive morphology. This reveals the importance of hybridization as a source of morphological variation, diversity and evolutionary novelty.

Novel colour polymorphisms in a hybrid zone ofMandarina(Gastropoda: Pulmonata)

A hybrid zone of the land snailsMandarina mandarinaandMandarina chichijimanain the oceanic Bonin Islands was studied morphologically and genetically to show the potential of hybridization as a source of morphological novelties. These species are clearly distinguishable on the basis of allozymes, colour polymorphisms, shell form and genital morphology, but exhibit a hybrid zone from the northeast to southwest of Chichijima island. There is a cline in the frequency of the colour patterns characteristic of each of the species, and specimens with intermediate colour pattern on the shells appear in the hybrid zone. Not only specimens with colour patterns that are overlapping of the patterns of the two species, but also specimens with unique colour patterns appear in the hybrid populations. These unique colour patterns are not found in the pure populations of both species or otherMandarinaspecies in Chichijima Islands, and it is suggested that these are produced by the hybridization. Because of the appearance of many types of unique colour patterns, variability of the colour polymorphism in the hybrid populations are remarkably higher than that in the pure populations. This result suggests that the novel morphology is produced by the hybridization between species with distinctive morphology. This reveals the importance of hybridization as a source of morphological variation, diversity and evolutionary novelty.

Hox gene expression in teleost fins and the origin of vertebrate digits.

Hox genes are essential for growth and patterning of the tetrapod limb skeleton. Mice mutant for the Hoxd-13 gene have an important delay in morphogenesis owing to reduced proliferation. Based on the appearance of atavisms in such mice, we suggested that modifications of Hox gene regulation may have been a source of morphological variation during the evolution of tetrapod limbs. Pectoral and pelvic fins are homologous to fore- and hindlimbs, respectively. To compare the relative importance of Hox genes during fin versus limb morphogenesis, we cloned zebrafish (Danio rerio) HoxD and HoxA complex genes and analysed their expression during fin development. The results suggest a scheme for the fin-limb transition in which the distal autopods (digits) are neomorphic structures produced by unequal proliferation of the posterior part of an ancestral appendix.

Labile sex expression in andromonoecious Solanum hirtum: pattern of variation in floral structure

Comparison of hermaphrodite and staminate flowers of andromonoecious Solanum hirtum must consider two sources of morphological variation: variation due to differences in flower position and variation due to conditions that affect sex expression. In S. hirtum, positional variation occurs among hermaphrodite flowers of unpollinated plants; flowers vary quantitatively but not functionally with position on the inflorescence. On inflorescences of pollinated, fruit-bearing plants, flowers vary both quantitatively and functionally with position such that flowers borne distally are functionally staminate. Corolla, anther, style, and ovary length of staminate flowers are significantly smaller than those of hermaphrodite flowers and they have fewer, smaller ovules. Failure of fruit set in staminate flowers is due to a combination of factors. Stigmas and styles of staminate flowers are reduced and appear to lack the secretions necessary for normal pollen germination and growth. In the ovules, the integumentary tapetum is collapsed and appears senescent and most megagametophytes are abnormal. Lack of pollen tube growth and abnormal ovules could both contribute to gynoecial sterility in staminate flowers. Key words: andromonoecy, floral structure, reproductive anatomy, sex expression, Solanum.