Does death drive the scaling of life?

The magnitude of many kinds of biological structures and processes scale with organismal size, often in regular ways that can be described by power functions. Traditionally, many of these "biological scaling" relationships have been explained based on internal geometric, physical, and energetic constraints according to universal natural laws, such as the "surface law" and "3/4-power law". However, during the last three decades it has become increasingly apparent that biological scaling relationships vary greatly in response to various external (environmental) factors. In this review, I propose and provide several lines of evidence supporting a new ecological perspective that I call the "mortality theory of ecology" (MorTE). According to this viewpoint, mortality imposes time limits on the growth, development, and reproduction of organisms. Accordingly, small, vulnerable organisms subject to high mortality due to predation and other environmental hazards have evolved faster, shorter lives than larger, more protected organisms. A MorTE also includes various corollary, size-related internal and external causative factors (e.g. intraspecific resource competition, geometric surface area to volume effects on resource supply/transport and the protection of internal tissues from environmental hazards, internal homeostatic regulatory systems, incidence of pathogens and parasites, etc.) that impact the scaling of life. A mortality-centred approach successfully predicts the ranges of body-mass scaling slopes observed for many kinds of biological and ecological traits. Furthermore, I argue that mortality rate should be considered the ultimate (evolutionary) driver of the scaling of life, that is expressed in the context of other proximate (functional) drivers such as information-based biological regulation and spatial (geometric) and energetic (metabolic) constraints.

Neurons with larval synaptic targets pioneer the later nervous system in the annelid Malacoceros fuliginosus

Comparative studies on the development of nervous systems have a significant impact on understanding animal nervous system evolution. Nevertheless, an important question is to what degree neuronal structures, which play an important role in early stages, become part of the adult nervous system or are relevant for its formation. This is likely in many direct developers, but it is not the case in forms with catastrophic metamorphosis. It is not clear in many forms with gradual metamorphosis. This introduces uncertainty in tracing the evolution of nervous systems and of larval forms. One of the prominent larval characteristics of numerous planktonic marine organisms is the epidermal ciliation used for swimming and steering, which disappears during metamorphosis. Therefore, the neuronal elements controlling the ciliary beating are often assumed to vanish with the cilia and regarded as purely larval specializations. With volume EM, we followed the neuronal targets of the very first pioneer neurons at the apical and posterior ends of the larva of the annelid Malacoceros fuliginosus. We observed that all of these pioneers appear to have a dual function. Although they are laying the paths for the later adult nervous system, they also make synaptic contact with the main ciliated ring of the larva. We propose that the formation of the later adult nervous system and the innervation of the larval locomotory organ are indeed closely linked to each other. This has implications for understanding the early nervous system development of marine larvae and for existing hypotheses on nervous system evolution.

Evolution of Maternal Provisioning and Development in the Ophiuroidea: Egg Size, Larval Form, and Parental Care

The Ophiuroidea is the most speciose class of echinoderms and has the greatest diversity of larval forms, but we know less about the evolution of development (evo-devo) in this group than for the other echinoderm classes. As is typical of echinoderms, evo-devo in the Ophiuroidea resulted in the switch from production of small eggs and feeding (planktotrophic) larvae to large eggs and non-feeding (lecithotrophic) larvae. Parental care (ovoviviparity or viviparity/matrotrophy) is the most derived life history. Analysis of egg data for 140 species (excluding viviparity and facultative planktotrophy) indicated a bimodal distribution in egg volume corresponding to planktotrophy and lecithotrophy + ovoviviparity, with three significant egg size groups due to the very large eggs of the ovoviviparous species. The marked reduction in fecundity in species with extremely large eggs is exemplified by the ovoviviparous species. Egg size in the two species with facultative planktotrophy was intermediate with respect to the two modes. Identifying the ancestral larval life history pattern and the pathways in the switch from feeding to non-feeding larvae is complicated by the two patterns of metamorphosis seen in species with planktotrophic development: Type I (ophiopluteus only) and Type II (ophiopluteus + vitellaria larva). The variability in arm resorption at metamorphosis across ophiuroid families indicates that the Type I and II patterns may be two ends of a morphological continuum. This variability indicates ancestral morphological plasticity at metamorphosis, followed by canalization in some taxa to the vitellaria as the metamorphic larva. Vestigial ophiopluteal traits in lecithotrophic ophioplutei and vitellaria indicate evolution from the ancestral (feeding larva) state. Parental care has evolved many times from an ancestor that had a planktonic ophiopluteus or vitellaria and is often associated with hermaphroditism and paedomorphosis. A secondary reduction in egg size occurred in the viviparous species.

Phoronida-A small clade with a big role in understanding the evolution of lophophorates

Phoronids, together with brachiopods and bryozoans, form the animal clade Lophophorata. Modern lophophorates are quite diverse-some can biomineralize while others are soft-bodied, they could be either solitary or colonial, and they develop through various eccentric larval stages that undergo different types of metamorphoses. The diversity of this clade is further enriched by numerous extinct fossil lineages with their own distinct body plans and life histories. In this review, I discuss how data on phoronid development, genetics, and morphology can inform our understanding of lophophorate evolution. The actinotrocha larvae of phoronids is a well documented example of intercalation of the new larval body plan, which can be used to study how new life stages emerge in animals with biphasic life cycle. The genomic and embryonic data from phoronids, in concert with studies of the fossil lophophorates, allow the more precise reconstruction of the evolution of lophophorate biomineralization. Finally, the regenerative and asexual abilities of phoronids can shed new light on the evolution of coloniality in lophophorates. As evident from those examples, Phoronida occupies a central role in the discussion of the evolution of lophophorate body plans and life histories.

Integrating Complex Life Cycles in Comparative Developmental Biology

The goal of comparative developmental biology is identifying mechanistic differences in embryonic development between different taxa and how these evolutionary changes have led to morphological and organizational differences in adult body plans. Much of this work has focused on direct-developing species in which the adult forms straight from the embryo and embryonic modifications have direct effects on the adult. However, most animal lineages are defined by indirect development, in which the embryo gives rise to a larval body plan and the adult forms by transformation of the larva. Historically, much of our understanding of complex life cycles is viewed through the lenses of ecology and zoology. In this review, we discuss the importance of establishing developmental rather than morphological or ecological criteria for defining developmental mode and explicitly considering the evolutionary implications of incorporating complex life cycles into broad developmental comparisons of embryos across metazoans.

Annelid functional genomics reveal the origins of bilaterian life cycles

Indirect development with an intermediate larva exists in all major animal lineages1, which makes larvae central to most scenarios of animal evolution2-11. Yet how larvae evolved remains disputed. Here we show that temporal shifts (that is, heterochronies) in trunk formation underpin the diversification of larvae and bilaterian life cycles. We performed chromosome-scale genome sequencing in the annelid Owenia fusiformis with transcriptomic and epigenomic profiling during the life cycles of this and two other annelids. We found that trunk development is deferred to pre-metamorphic stages in the feeding larva of O. fusiformis but starts after gastrulation in the non-feeding larva with gradual metamorphosis of Capitella teleta and the direct developing embryo of Dimorphilus gyrociliatus. Accordingly, the embryos of O. fusiformis develop first into an enlarged anterior domain that forms larval tissues and the adult head12. Notably, this also occurs in the so-called 'head larvae' of other bilaterians13-17, with which the O. fusiformis larva shows extensive transcriptomic similarities. Together, our findings suggest that the temporal decoupling of head and trunk formation, as maximally observed in head larvae, facilitated larval evolution in Bilateria. This diverges from prevailing scenarios that propose either co-option9,10 or innovation11 of gene regulatory programmes to explain larva and adult origins.

Worms and gills, plates and spines: the evolutionary origins and incredible disparity of deuterostomes revealed by fossils, genes, and development

Deuterostomes are the major division of animal life which includes sea stars, acorn worms, and humans, among a wide variety of ecologically and morphologically disparate taxa. However, their early evolution is poorly understood, due in part to their disparity, which makes identifying commonalities difficult, as well as their relatively poor early fossil record. Here, we review the available morphological, palaeontological, developmental, and molecular data to establish a framework for exploring the origins of this important and enigmatic group. Recent fossil discoveries strongly support a vermiform ancestor to the group Hemichordata, and a fusiform active swimmer as ancestor to Chordata. The diverse and anatomically bewildering variety of forms among the early echinoderms show evidence of both bilateral and radial symmetry. We consider four characteristics most critical for understanding the form and function of the last common ancestor to Deuterostomia: Hox gene expression patterns, larval morphology, the capacity for biomineralization, and the morphology of the pharyngeal region. We posit a deuterostome last common ancestor with a similar antero-posterior gene regulatory system to that found in modern acorn worms and cephalochordates, a simple planktonic larval form, which was later elaborated in the ambulacrarian lineage, the ability to secrete calcium minerals in a limited fashion, and a pharyngeal respiratory region composed of simple pores. This animal was likely to be motile in adult form, as opposed to the sessile origins that have been historically suggested. Recent debates regarding deuterostome monophyly as well as the wide array of deuterostome-affiliated problematica further suggest the possibility that those features were not only present in the last common ancestor of Deuterostomia, but potentially in the ur-bilaterian. The morphology and development of the early deuterostomes, therefore, underpin some of the most significant questions in the study of metazoan evolution.

Innovation Through Heterochrony: An Amphioxus Perspective on Telencephalon Origin and Function

Heterochrony has played a key role in the evolution of invertebrate larval types, producing “head larvae” in diverse taxa, where anterior structures are accelerated and specialized at the expense of more caudal ones. For chordates, judging from amphioxus, the pattern has been more one of repeated acceleration of adult features so that they function earlier in development, thus converting the ancestral larva, whether it was a head larva or not, into something progressively more chordate-like. Recent molecular data on gene expression patterns in the anterior nerve cord of amphioxus point to a similar process being involved in the origin of the telencephalon. As vertebrates evolved, a combination of acceleration and increasing egg size appears here to have allowed the development of a structure that would originally have emerged only gradually in the post-embryonic phase of the life history to be compressed into embryogenesis. The question then is what, in functional terms, makes the telencephalon so important to the survival of post-embryonic ancestral vertebrates that this was adaptively advantageous. A better understanding of the function this brain region performs in amphioxus may help provide the answer.