The ontogeny of disparity in Cupressaceae seed cones

Ontogenetic shape change has long been recognized to be important in generating patterns of morphological diversity and may be especially important in plant reproductive structures. We explore how seed cone disparity in Cupressaceae changes over ontogeny by comparing pollination-stage and mature cones. We sampled cones at pollen and seed release and measured cone scales using basic morphometric shape variables. We used multivariate statistical methods, particularly hypervolume overlap calculations, to measure morphospace occupation and disparity. Cone scales at both pollination and maturity exhibit substantial variability, although the disparity is greater at maturity. Mature cone scales are also more clustered in trait space, showing less overlap with other taxa than at pollination. These patterns reflect two growth strategies that generate closed cones over maturation, either through thin laminar scales or relatively thick, peltate scales, resulting in two distinct regions of morphospace occupation. Disparity patterns in Cupressaceae seed cones change over ontogeny, reflecting shifting functional demands that require specific patterns of cone scale growth. The evolution of Cupressaceae reproductive disparity therefore represents selection for trajectories of ontogenetic shape change, a phenomenon that should be widespread across seed plants.

Quantifying the complexity of plant reproductive structures reveals a history of morphological and functional integration

Vascular plant reproductive structures have undoubtedly become more complex through time, evolving highly differentiated parts that interact in specialized ways. But quantifying these patterns at broad scales is challenging because lineages produce disparate reproductive structures that are often difficult to compare and homologize. We develop a novel approach for analysing interactions within reproductive structures using networks, treating component parts as nodes and a suite of physical and functional interactions among parts as edges. We apply this approach to the plant fossil record, showing that interactions have generally increased through time and that the concentration of these interactions has shifted towards differentiated surrounding organs, resulting in more compact, functionally integrated structures. These processes are widespread across plant lineages, but their extent and timing vary with reproductive biology; in particular, seed-producing structures show them more strongly than spore or pollen-producing structures. Our results demonstrate that major reproductive innovations like the origin of seeds and angiospermy were associated with increased integration through greater interactions among parts. But they also reveal that for certain groups, particularly Mesozoic gymnosperms, millions of years elapsed between the origin of reproductive innovations and increased interactions among parts within their reproductive structures.

Neontological and paleontological congruence in the evolution of Podocarpaceae (coniferales) reproductive morphology

Introduction

Podocarpaceae are a diverse, primarily tropical conifer family that commonly produce large leaves and highly reduced, fleshy seed cones bearing large seeds. These features may result from relatively recent adaptation to closed-canopy angiosperm forests and bird-mediated seed dispersal, although determining precisely when shifts in leaf and seed cone morphology occurred is difficult due to a sparse fossil record and relatively few surviving deep lineages.

Methods

We compare the fossil record of Podocarpaceae with results from ancestral state reconstruction methods and correlated character models using neontological data and a previously published molecular time-tree.

Results

Ancestral state reconstructions suggest that small leaves, small seeds, and multi-seeded cones are ancestral in crown Podocarpaceae, with reduced cones bearing few seeds appearing in the Early Cretaceous and the correlated evolution of large leaves and large seeds occurring from the Late Cretaceous onwards. The exact timing of these shifts based on neontological data alone are poorly constrained, however, and estimates of leaf and seed size are imprecise.

Discussion

The fossil record is largely congruent with results based on the molecular time-tree, but provide important constraints on the range of leaf and seed sizes that were present in Cretaceous Podocarpaceae and the time by which changes in cone morphology and seed size likely occurred. We suggest in particular that reduced seed cones appeared in the Early Cretaceous and are linked to the contemporaneous diversification of small bodied avialans (birds), with shifts to larger seed sizes occurring after the Cretaceous in association with the spread of closed-canopy angiosperm forests.

How many ways can you build a conifer cone? A commentary on 'Origin and evolution of Podocarpaceae seed cones'

This article comments on:

Raees Khan, Robert S. Hill, Veit M. Dörken and Ed Biffin, Detailed seed cone morpho-anatomy of the Prumnopityoid clade: an insight into the origin and evolution of Podocarpaceae seed cones, Annals of Botany, Volume 130, Issue 5, 1 November 2022, Pages 637–655 https://doi.org/10.1093/aob/mcac097

Reproductive innovations and pulsed rise in plant complexity

[Figure: see text].

A novel reproductive strategy in an Early Devonian plant

Bonacorsi et al. describe a new fossil from the Early Devonian that provides the earliest clear evidence for more advanced reproductive biology in land plants. The plant produced multiple spore size classes, which is an essential innovation necessary for all advanced plant reproductive strategies, including seeds and flowers.

Early Cretaceous abietoid Pinaceae from Mongolia and the history of seed scale shedding

PREMISE

Seed cones of extant Pinaceae exhibit two mechanisms of seed release. In "flexers" the cone scales remain attached to the central axis, while flexing and separating from each other to release the seeds. In "shedders" scales are shed from the axis, with the seeds either remaining attached to the scale or becoming detached. The early fossil history of Pinaceae from the Jurassic to Early Cretaceous is dominated by flexing seed cones, while the systematic information provided by shedding fossil cones has been overlooked and rarely integrated with data based on compression and permineralized specimens. We describe the earliest and best-documented evidence of a "shedder" seed cone from the Aptian-Albian of Mongolia.

METHODS

Lignite samples from Tevshiin Govi locality were disaggregated in water, washed, and dried in air. Fossils were compared to material of extant Pinaceae using LM and CT scans.

RESULTS

Lepidocasus mellonae gen. et sp. nov. is characterized by a seed cone that disarticulated at maturity and shed obovate bract-scale complexes that have a distinctive ribbed surface and an abaxial surface covered with abundant trichomes. The ovuliferous scale has ca. 30-40 resin canals, but only scarce xylem near the attachment to the cone axis. Resin vesicles are present in the seed integument. Phylogenetic analysis places Lepidocasus as sister to extant Cedrus within the abietoid grade.

CONCLUSIONS

The exquisite preservation of the trichomes in L. mellonae raises questions about their potential ecological function in the cones of fossil and living Pinaceae. Lepidocasus mellonae also shows that a shedding dispersal syndrome, a feature that has often been overlooked, evolved early in the history of Pinaceae during the Early Cretaceous.

Accumulation over evolutionary time as a major cause of biodiversity hotspots in conifers

Biodiversity hotspots are important for understanding how areas of high species richness form, but disentangling the processes that produce them is difficult. We combine geographical ranges, phylogenetic relationships and trait data for 606 conifer species in order to explore the mechanisms underlying richness hotspot formation. We identify eight richness hotspots that overlap known centres of plant endemism and diversity, and find that conifer richness hotspots occur in mountainous areas within broader regions of long-term climate stability. Conifer hotspots are not unique in their species composition, traits or phylogenetic structure; however, a large percentage of their species are not restricted to hotspots and they rarely show either a preponderance of new radiating lineages or old relictual lineages. We suggest that conifer hotspots have primarily formed as a result of lineages accumulating over evolutionary time scales in stable mountainous areas rather than through high origination, preferential retention of relictual lineages or radiation of species with unique traits, although such processes may contribute to nuanced differences among hotspots. Conifers suggest that a simple accumulation of regional diversity can generate high species richness without additional processes and that geography rather than biology may play a primary role in hotspot formation.

Reproductive Ontogeny and the Evolution of Morphological Diversity in Conifers and Other Plants

Biologists often study morphological evolution through form and function relationships. But biological structures can perform multiple functional roles, complicating efforts to understand the evolutionary significance of any one relationship. Plant reproductive organs perform multiple roles in a sequence, however, which provides a unique opportunity to understand how structures evolve to meet multiple functional demands. Using conifers as a study group, we discuss how a shared developmental trajectory links the performance of sequential functional roles. Variation in development among lineages can underlie morphological diversity; pollination-stage seed cones in Pinaceae conifers function similarly but show diverse forms reflecting differences in developmental rate. As cones develop further, the morphologies that they use to perform later functional roles are influenced by the specific developmental patterns used to meet earlier demands, which may ultimately limit morphological diversity. However, we also show how selective pressures relating to the final functional stage (seed dispersal) may influence cone anatomy and morphology over all previous stages, highlighting the complex linkages among form, function, and development. We end by discussing the potential relationships between functional ontogeny and morphological disparity in plant reproductive structures more broadly, suggesting that the complex functional roles associated with seed plant reproduction probably underlie the high disparity in this group.

Not all 'pine cones' flex: functional trade-offs and the evolution of seed release mechanisms

Seed dispersal is critical for plants, but the evolution of mechanisms that actually release seeds from their parents is not well understood. We use the reproductive cones of conifers, specifically the Pinaceae clade, to explore the factors driving the evolution of different release mechanisms in plants. We combine comparative anatomical and phylogenetic analyses to test whether fundamental trade-offs in the mechanical and hydraulic properties of vasculature underlie the evolution of two seed release mechanisms: cone scale flexion and cone scale shedding. We then test whether these mechanisms are linked with differences in seed size, dispersal syndrome and reproductive allocation. Cone scale xylem in flexing species is tough, but poorly conductive. Xylem in shedding species is less extensive, fragile and highly conductive; its thin-walled tracheids allow scales to easily fracture at maturity. Shedding is also consistently associated with large, densely packed seeds. Pinaceae cones exploit a well-known trade-off in xylem mechanical strength vs hydraulic efficiency to generate release mechanisms that allow seeds of various sizes to leave the protecting cone. The linkage among release mechanisms, vascular anatomy and seed traits illustrates how a wide variety of selective pressures may influence the function and physiology of reproductive structures.

Functional diversity and convergence in the evolution of plant reproductive structures

BACKGROUND AND AIMS

Structures that simultaneously perform many functional roles are likely to show a variety of morphological solutions to these demands, and thus probably exhibit high morphological disparity. In contrast, specialization for a few simple functions should result in a more limited suite of morphologies. We explore this idea using lycopsid reproductive structures, which, throughout their history, have performed a limited set of functional roles compared with the reproductive structures of other plant groups such as seed plants.

METHODS

We scored living and fossil lycopsid taxa for 18 discrete character measurements and several continuous traits, including sporangium size, supporting axis diameter, and strobilus length and width. We used the discrete characters to construct a multivariate morphospace for lycopsid reproductive morphology through time, and the continuous characters to test whether fossil and extant lycopsids show similar patterns of tissue allocation within reproductive structures.

RESULTS

Lycopsids occupy similar areas of reproductive morphospace and show similar patterns of tissue allocation over most of their history, alternating between diffuse fertile zones with leaf-like sporophylls and compact strobili with specialized sporophylls that allow sporangia to be closely packed while also protected during their development. Growth habit also plays an important role in lycopsid reproductive evolution, broadly influencing the size and shape of reproductive structures.

CONCLUSIONS

Lycopsid reproductive structures are primarily specialized for densely packaging sporangia, and are consistent with the idea that performing limited functional roles is associated with reduced morphological disparity. Morphologies similar to lycopsid strobili are also found in other groups with simple, wind-dispersed propagules, suggesting that the same processes occur across plant lineages.

An overview of extant conifer evolution from the perspective of the fossil record

PREMISE OF THE STUDY

Conifers are an important living seed plant lineage with an extensive fossil record spanning more than 300 million years. The group therefore provides an excellent opportunity to explore congruence and conflict between dated molecular phylogenies and the fossil record.

METHODS

We surveyed the current state of knowledge in conifer phylogenetics to present a new time-calibrated molecular tree that samples ~90% of extant species diversity. We compared phylogenetic relationships and estimated divergence ages in this new phylogeny with the paleobotanical record, focusing on clades that are species-rich and well known from fossils.

KEY RESULTS

Molecular topologies and estimated divergence ages largely agree with the fossil record in Cupressaceae, conflict with it in Araucariaceae, and are ambiguous in Pinaceae and Podocarpaceae. Molecular phylogenies provide insights into some fundamental questions in conifer evolution, such as the origin of their seed cones, but using them to reconstruct the evolutionary history of specific traits can be challenging.

CONCLUSIONS

Molecular phylogenies are useful for answering deep questions in conifer evolution if they depend on understanding relationships among extant lineages. Because of extinction, however, molecular datasets poorly sample diversity from periods much earlier than the Late Cretaceous. This fundamentally limits their utility for understanding deep patterns of character evolution and resolving the overall pattern of conifer phylogeny.

Why are the seed cones of conifers so diverse at pollination?

BACKGROUND AND AIMS

Form and function relationships in plant reproductive structures have long fascinated biologists. Although the intricate associations between specific pollinators and reproductive morphology have been widely explored among animal-pollinated plants, the evolutionary processes underlying the diverse morphologies of wind-pollinated plants remain less well understood. Here we study how this diversity may have arisen by focusing on two conifer species in the pine family that have divergent reproductive cone morphologies at pollination.

METHODS

Standard histology methods, artificial wind pollination assays and phylogenetic analyses were used in this study.

KEY RESULTS

A detailed study of cone ontogeny in these species reveals that variation in the rate at which their cone scales mature means that pollination occurs at different stages in their development, and thus in association with different specific morphologies. Pollination experiments nevertheless indicate that both species effectively capture pollen.

CONCLUSIONS

In wind-pollinated plants, morphological diversity may result from simple variation in development among lineages rather than selective pressures for any major differences in function or performance. This work also illustrates the broader importance of developmental context in understanding plant form and function relationships; because plant reproductive structures perform many different functions over their lifetime, subtle differences in development may dramatically alter the specific morphologies that they use to meet these demands.

Variation in seed size is structured by dispersal syndrome and cone morphology in conifers and other nonflowering seed plants

Seed size varies tremendously in plants and its evolution is influenced by multiple ecological and biological factors that are difficult to disentangle. In this study, we focus on understanding the role of seed dispersal by animals in the evolution of seed size in conifers, the most diverse extant nonflowering seed plant group. Relationships among seed size, dispersal syndrome, climate and cone morphology were analyzed across conifers using quantitative models of character evolution and phylogenetic regression techniques. Dispersal syndrome is a more consistent predictor of seed size within major extant conifer clades than climate. Seeds are generally larger in animal-dispersed than wind-dispersed species, and particular cone morphologies are consistently associated with specific ranges in seed size. Seed size and cone morphology evolve in a correlated manner in many animal-dispersed conifers, following a trade-off that minimizes the total size of the dispersal unit. These relationships are also present in other nonflowering seed plant groups, and have been important in the evolution of seeds and cones at least over the Cenozoic and perhaps over much of the later Mesozoic.

Fossils matter: improved estimates of divergence times in Pinus reveal older diversification

BACKGROUND

The taxonomy of pines (genus Pinus) is widely accepted and a robust gene tree based on entire plastome sequences exists. However, there is a large discrepancy in estimated divergence times of major pine clades among existing studies, mainly due to differences in fossil placement and dating methods used. We currently lack a dated molecular phylogeny that makes use of the rich pine fossil record, and this study is the first to estimate the divergence dates of pines based on a large number of fossils (21) evenly distributed across all major clades, in combination with applying both node and tip dating methods.

RESULTS

We present a range of molecular phylogenetic trees of Pinus generated within a Bayesian framework. We find the origin of crown Pinus is likely up to 30 Myr older (Early Cretaceous) than inferred in most previous studies (Late Cretaceous) and propose generally older divergence times for major clades within Pinus than previously thought. Our age estimates vary significantly between the different dating approaches, but the results generally agree on older divergence times. We present a revised list of 21 fossils that are suitable to use in dating or comparative analyses of pines.

CONCLUSIONS

Reliable estimates of divergence times in pines are essential if we are to link diversification processes and functional adaptation of this genus to geological events or to changing climates. In addition to older divergence times in Pinus, our results also indicate that node age estimates in pines depend on dating approaches and the specific fossil sets used, reflecting inherent differences in various dating approaches. The sets of dated phylogenetic trees of pines presented here provide a way to account for uncertainties in age estimations when applying comparative phylogenetic methods.

Early Cretaceous Umkomasia from Mongolia: implications for homology of corystosperm cupules

Corystosperms, a key extinct group of Late Permian to Early Cretaceous plants, are important for understanding seed plant phylogeny, including the evolution of the angiosperm carpel and anatropous bitegmic ovule. Here, we describe a new species of corystosperm seed-bearing organ, Umkomasia mongolica sp. nov., based on hundreds of three-dimensionally preserved mesofossils from the Early Cretaceous of Mongolia. Individual seed-bearing units of U. mongolica consist of a bract subtending an axis that bifurcates, with each fork (cupule stalk) bearing a cupule near the tip. Each cupule is formed by the strongly reflexed cupule stalk and two lateral flaps that partially enclose an erect seed. The seed is borne at, or close to, the tip of the reflexed cupule stalk, with the micropyle oriented towards the stalk base. The corystosperm cupule is generally interpreted as a modified leaf that bears a seed on its abaxial surface. However, U. mongolica suggests that an earlier interpretation, in which the seed is borne directly on an axis (shoot), is equally likely. The 'axial' interpretation suggests a possible relationship of corystosperms to Ginkgo. It also suggests that the cupules of corystosperms may be less distinct from those of Caytonia than has previously been supposed.

Integration and macroevolutionary patterns in the pollination biology of conifers

Integration influences patterns of trait evolution, but the relationship between these patterns and the degree of trait integration is not well understood. To explore this further, we study a specialized pollination mechanism in conifers whose traits are linked through function but not development. This mechanism depends on interactions among three characters: pollen that is buoyant, ovules that face downward at pollination, and the production of a liquid droplet that buoyant grains float through to enter the ovule. We use a well-sampled phylogeny of conifers to test correlated evolution among these characters and specific sequences of character change. Using likelihood models of character evolution, we find that pollen morphology and ovule characters evolve in a concerted manner, where the flotation mechanism breaks down irreversibly following changes in orientation or drop production. The breakdown of this functional constraint, which may be facilitated by the lack of developmental integration among the constituent traits, is associated with increased trait variation and more diverse pollination strategies. Although this functional "release" increases diversity in some ways, the irreversible way in which the flotation mechanism is lost may eventually result in its complete disappearance from seed plant reproductive biology.

Cone size is related to branching architecture in conifers

The relationship between branch diameter and leaf size has been widely used to understand how vegetative resources are allocated in plants. Branching architecture influences reproductive allocation as well, but fewer studies have explored this relationship at broad phylogenetic or ecological scales. In this study, we tested whether pollen-producing and seed-producing cone size scales with branch diameter in conifers, a diverse and globally distributed lineage of nonflowering seed plants. Branch diameter and cone size were analyzed using multiple regression models and evolutionary models of trait evolution for a data set of 293 extant conifer species within an explicit phylogenetic framework. Branch diameter is a strong predictor of cone size across conifer species, particularly for pollen cones and dry seed cones. However, these relationships are complex in detail because leaf morphology and seed dispersal biology influence the specific ways in which they are expressed. The ubiquity and strength of these scaling relationships across conifers suggest that reproductive and vegetative morphologies are coupled in the group, and it is therefore difficult to disentangle the evolution of cone size from the evolution of branching architecture.

Pinaceae-like reproductive morphology in Schizolepidopsis canicularis sp. nov. from the Early Cretaceous (Aptian-Albian) of Mongolia

PREMISE OF THE STUDY

Seed cone scales assigned to the genus Schizolepidopsis are widespread in Late Triassic to Cretaceous Eurasian deposits. They have been linked to the conifer family Pinaceae based on associated vegetative remains, but their exact affinities are uncertain. Recently discovered material from the Early Cretaceous of Mongolia reveals important new information concerning Schizolepidopsis cone scales and seeds, and provides support for a relationship between the genus and extant Pinaceae.

METHODS

Specimens were collected from Early Cretaceous (probable Aptian-Albian) lignite deposits in central Mongolia. Lignite samples were disaggregated, cleaned in hydrofluoric acid, and washed in water. Specimens were selected for further study using light and electron microscopy.

KEY RESULTS

Schizolepidopsis canicularis seed cones consist of loosely arranged, bilobed ovulate scales subtended by a small bract. A single inverted seed with an elongate micropyle is borne on each lobe of the ovulate scale. Each seed has a wing formed by the separation of the adaxial surface of the ovulate scale.

CONCLUSIONS

Schizolepidopsis canicularis produced winged seeds that formed in a manner that is unique to Pinaceae among extant conifers. We do not definitively place this species in Pinaceae pending more complete information concerning its pollen cones and vegetative remains. Nevertheless, this material suggests that Schizolepidopsis may be important for understanding the early evolution of Pinaceae, and may potentially help reconcile the appearance of the family in the fossil record with results based on phylogenetic analyses of molecular data.

Explaining the distribution of breeding and dispersal syndromes in conifers

The evolution of plants exhibiting different sexes, or dioecy, is correlated with a number of ecological and life-history traits such as woody growth form and animal-dispersed seeds, but the underlying causes of these associations are unclear. Previous work in seed plants has suggested that the evolution of fleshy cones or seeds may favour dioecy. In this study, we use a well-sampled molecular phylogeny of conifers to show that although dioecy and fleshiness strongly co-occur at the species level, this relationship has not resulted from numerous separate origins of this trait combination or from differential rates of diversification. Instead, we suggest that two character combinations-the ancestral dry-monoecious condition and the derived fleshy-dioecious condition-have persisted in conifers longer than other combinations over evolutionary time. The persistence of these trait combinations appears to reflect differences in the rate of successful transition into and out of these character states over time, as well as the geographical restriction of species with rare combinations and their consequent vulnerability to extinction. In general, we argue that such persistence explanations should be considered alongside 'key innovation' hypotheses in explaining the phylogenetic distribution of traits.

Branching habit and the allocation of reproductive resources in conifers

BACKGROUND AND AIMS

Correlated relationships between branch thickness, branch density, and twig and leaf size have been used extensively to study the evolution of plant canopy architecture, but fewer studies have explored the impact of these relationships on the allocation of reproductive resources. This study quantifies pollen cone production in conifers, which have similar basic reproductive biology but vary dramatically in branching habit, in order to test how differences in branch diameter influence pollen cone size and the density with which they are deployed in the canopy.

METHODS

Measurements of canopy branch density, the number of cones per branch and cone size were used to estimate the amount of pollen cone tissues produced by 16 species in three major conifer clades. The number of pollen grains produced was also estimated using direct counts from individual pollen cones.

KEY RESULTS

The total amount of pollen cone tissues in the conifer canopy varied little among species and clades, although vegetative traits such as branch thickness, branch density and pollen cone size varied over several orders of magnitude. However, branching habit controls the way these tissues are deployed: taxa with small branches produce small pollen cones at a high density, while taxa with large branches produce large cones relatively sparsely.

CONCLUSIONS

Conifers appear to invest similar amounts of energy in pollen production independent of branching habit. However, similar associations between branch thickness, branch density and pollen cone size are seen across conifers, including members of living and extinct groups not directly studied here. This suggests that reproductive features relating to pollen cone size are in large part a function of the evolution of vegetative morphology and branching habit.

Predation and protection in the macroevolutionary history of conifer cones

Conifers are an excellent group in which to explore how changing ecological interactions may have influenced the allocation of reproductive tissues in seed plants over long time scales, because of their extensive fossil record and their important role in terrestrial ecosystems since the Palaeozoic. Measurements of individual conifer pollen-producing and seed-producing cones from the Pennsylvanian to the Recent show that the relative amount of tissue invested in pollen cones has remained constant through time, while seed cones show a sharp increase in proportional tissue investment in the Jurassic that has continued to intensify to the present day. Since seed size in conifers has remained similar through time, this increase reflects greater investment in protective cone tissues such as robust, tightly packed scales. This shift in morphology and tissue allocation is broadly concurrent with the appearance of new vertebrate groups capable of browsing in tree canopies, as well as a diversification of insect-feeding strategies, suggesting that an important change in plant-animal interactions occurred over the Mesozoic that favoured an increase in seed cone protective tissues.

Flotation preferentially selects saccate pollen during conifer pollination

• Among many species of living conifers the presence of pollen with air bladders (saccate pollen) is strongly associated with downward-facing ovules and the production of pollination drops. This combination of features enables saccate pollen grains captured in the pollination drop to float upwards into the ovule. Despite the importance of this mechanism in understanding reproduction in living conifers and in extinct seed plants with similar morphologies, experiments designed to test its effectiveness have yielded equivocal results. • In vitro and in vivo pollination experiments using saccate and nonsaccate pollen were performed using modeled ovules and two Pinus species during their natural pollination period. • Buoyant saccate pollen readily floated through aqueous droplets, separating these grains from nonbuoyant pollen and spores. Ovules that received saccate pollen, nonsaccate pollen or a mixture of both all showed larger amounts and higher proportions of saccate pollen inside ovules after drop secretion. • These results demonstrate that flotation is an effective mechanism of pollen capture and transport in gymnosperms, and suggest that the prevalence of saccate grains and downward-facing ovules in the evolutionary history of seed plants is a result of the widespread use of this mechanism.