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1.1: What is EvoDevo? - Biology

1.1: What is EvoDevo? - Biology


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Anomalies

I first became interested in Developmental Biology while volunteering in a special needs classroom. There I saw a wider variety of human morphology and behavior than I was normally exposed to. At the time, I wondered how these differences came about. This led me to the fields of Genetics and Developmental Biology, I got my start in a Clinical Genetics lab where I assisted in diagnosing genetic disorders. Couples with a familial history of a particular disorder would get a blood draw, we would take the blood, extract DNA, and perform SNP or RFLP testing to genotype it. We would then make a prediction regarding the likelihood that their future child would have or be a carrier of a particular disorder. The lab next to mine studied teratogenesis, or the development of birth disorders. Couples that used that clinical lab came bearing abnormal bloodwork and ultrasounds. Our labs were two sides of the same coin: we were interested in anomalies.

Anomalies refer to anything outside the ordinary, for example conjoined twins or missing limbs. Even though we commonly think of developmental anomalies as inherently "bad", they are not fundamentally so. For example, the average height of an NBA player is 6'7". Although this is anomalous (only 0.045% or one in 2200 American men are this tall or taller), it is not considered at all detrimental to be this tall. If you are a Developmental Biologist, you relish natural variation as a window into developmental processes. For example, you might find out what an NBA player ate growing up, how tall his parents are, if he grew up in a rural area or the city, and compare these data to the data of an average American man. The differences between the two can help you generate hypotheses regarding the developmental processes leading to tall height.

Figure 1: The poster to the left shows an advertisement for a tall man, Franz Winklemeier, who toured Europe in the 1880s. Image from Wellcome Images CC BY 4.0

At a deeper level we also notice something more fundamentally interesting about a very tall (or very short) person. Most people who are quite tall or quite short (say a 4'7" woman), have bodily proportions that are nearly the same as someone of average height. For example, their arms aren't placed too low, their neck isn't particularly long, their legs reach all the way to the ground. In short, their body has been "scaled" to a larger or smaller size. This leads us to think that perhaps development itself is scalable. Surely the blueprints for building a very short person are not so different from the blueprints for building a very tall person. Our current understanding is that signals from growing body parts signal to each other to accomplish this task. The same signals build a short or tall body, but a tall body might undergo more cell division or longer periods of growth than a short body.

Classical developmental biology relies on mutations (gene based) and teratogenic (environment based) anomalies to dissect processes like this. Not only can we compare "normal" anomalies like heights that fall two standard deviations from the average, but we can also look at more rare anomalies like conjoined twin tadpoles or a cyclopic (one eyed) mouse. Both of these anomalies have the bonus of being able to be created in the lab. In fact, we can create them using either genetics, or environmental insult (a teratogen).

The Evolution of Development or the Development of Evolution?

In a very real way, the fields of Development and Evolution cannot be truly separated. When we study Developmental Biology we are mostly looking at a fine-tuned mechanical and genetic process that has been selected on for eons. Not only can evolution select on the final product - a working, fertile adult - but it also can act at each developmental stage. It is easy to see how evolution acts through natural selection on adults, but how can it act on development itself? Consider the useful but pesky fruit fly, Drosophila melanogaster. Unlike you and me, who have support from our society (family, friends, institutions), Drosophila are r-selected. They survive by mating frequently and producing massive amounts of offspring that they leave behind on rotting fruit. Selection on adult Drosophila includes things such as ability to fly, to find food, find mates, and evade fly swatters. But further types of selection affect embryonic Drosophila. Since the ability to mate quickly and produce many offspring is such a strong selective pressure on this species, embryos have evolved rapid development in response. In fact, Drosophila undergo such unusual developmental patterning in order to accomplish this incredibly fast development (about 24 hours to hatch into a larva), that we can use them as a model to see how evolution can speed up normal developmental processes. Surprisingly, to me at least, speeding up development doesn't just mean that everything moves along more quickly. Instead, Drosophila and its relatives undergo a unique type of embryonic development called "long germ-band" development. Instead of developing by slowly elongating and adding new differentiated tissue to the posterior end of the body, as many animals do, long germ-band insects develop all at once - rapidly subdividing an undifferentiated field of cells into the many parts of the larval body.

This example is a classic illustration of evolution acting on developmental processes. There are many other ways evolution acts on development, these are often related to the physical constraints experienced by a growing embryo. On the flip side, we can also think of evolution itself as being constrained by development. Although much of natural selection acts on adults, that adult body must be formed by developmental processes inherited from ancestors and influenced by the environment. This is a limit on the types of bodies that can be produced, even if selective pressures are strong. However, the average member of a species does not show us the extremes of what morphologies are possible given evolutionary and physical constraints. This is where it becomes important to look at anomalies and other variation. When we see that height is scalable, we can look at the evolution of height in the past (what pressures drove the Homo genus to become tall with Homo erectus and its descendants), pressures on height in the present (locomoter issues with very short or very tall heights), as well as the development of height (length of puberty, environmental factors such as food and toxins). By examining our very short and very tall people, we can see the outer limits of these evolutionary and developmental constraints. Likewise by looking at rare anomalies (for example people born without limbs) we can examine the limits of perturbing development and find out what is possible.

One great example of the interconnectedness of evolution and development is the problem of scaling in flies. Briefly, there are closely related fly genera that vary widely in size but have very similar proportions. An outstanding question in EvoDevo is: how do flies change their scale by tweaking or changing developmental processes?


Introduction

Ctenophores (also known as comb jellies or sea gooseberries) are free-living marine organisms. They represent a non-bilaterian lineage of Metazoa (besides cnidarians, sponges, and placozoans) of particular importance for understanding early animal evolution. Most ctenophores look like ghostly, transparent jellies with eight comb rows of iridescent, compound cilia used in swimming, and many have long, retractile tentacles with a comb row of side branches (Figure 1). The side branches of the tentacles are covered with colloblasts cells which contain vesicles with a sticky substance used in capturing prey organisms, such as copepods. The rather familiar ‘sea gooseberries’ (Pleurobrachia) probably represent an ancestral type, which is pelagic and shows almost the same morphology in both the adult and in the newly hatched cydippid stage. All described ctenophore species hatch as a cydippid stage (except the highly specialized Beroe, which lacks tentacles in all stages) but adults of the different species show enormous variation in form. One line of morphological specialization in adult ctenophores involves enlargement of the oral lobes and gradual loss of the tentacles, as seen in Mnemiopsis. A number of open-ocean species have very large oral lips and only small comb rows and are extremely fragile. Beroe lacks tentacles and feeds on other ctenophores or medusae, which may be swallowed whole. Beroe also has the remarkable ability to ‘bite’ into jellies with their macrociliary ‘teeth’. A specialized group has a flattened benthic adult stage that crawls on the ciliated extended lips of the wide mouth and extends the tentacles through small funnels formed by lateral folds of the mouth this group lacks the hallmark comb rows. Some of these species are brightly colored and are found living on other animals, such as corals or sea stars. Newly hatched juveniles produce sperm and a number of small eggs, which develop into normal juveniles. After a period of growth, the small individuals become sexually mature again and produce numerous larger eggs.

Ctenophore diversity. A, Pleurobrachia bachei (D Kent) B, Mnemiopsis leidyi (K Brandt) C, Beroe gracilis with Pleurobrachia pileus in the stomach (C Marneff) D, Thalassocalyce inconstans (L Madin) E, Coeloplana astericola (yellow-brown with thin, branched tentacles) on the red sea-star Echinaster luzonicus (D Fugitt). Reproduced with permission from D Kent, K Brandt, C Marneff, L Madin and D Fugitt.

Unlike sponges, ctenophores have a gut with digestive enzymes lined with an epithelium, a complex nervous system and a complicated system of muscles [1]. The ctenophore nervous system is organized into an epithelial and a mesogleal nerve net and two parallel nerve cords in the tentacles [1]. They have sophisticated sensory cells, including putative photo-, mechano- and gravi-receptors [2, 3]. The nervous system controls the activity of cilia, bioluminescent flashes and muscular contractions [3, 4]. Ctenophores use ciliary comb rows for locomotion and the beating frequency and the arrests of the cilia are controlled by dedicated neuronal systems [3]. The cydippid Euplokamis has giant axons that run longitudinally along the eight comb rows and control fast backward and forward escape responses [5].

Ctenophores have been classified as eumetazoans, often as the sister group of the cnidarians, but also as the sister group of the bilaterians. Two recent whole-genome analyses of the ctenophores Mnemiopsis leidyi [6] and Pleurobrachia bachei [7] have indicated, however, that the ctenophores do not belong to the Eumetazoa but are instead the sister group to all other metazoans, a position more typically occupied by the sponges. The placement of ctenophores as a sister to all other metazoans was supported by earlier phylogenetic studies [8–12], while other work indicated sponges as the sister lineage to all other metazoans [13–16]. The recent analyses of ctenophore genomes also support the non-canonical phylogeny based on the absence from ctenophores of key eumetazoan characters, such as Hox genes and microRNAs. If this phylogeny is correct, then nerves and muscles must either have evolved independently in Ctenophora and Eumetazoa (for simplicity, referring to Cnidaria plus Bilateria throughout the paper), or these systems evolved in the metazoan ancestor and have been lost in sponges and placozoans, lineages without any trace of synaptically connected nerve cells. Alternatively, ctenophores may be a sister group to cnidarians or to bilaterians or to eumetazoans, and their placement outside the eumetazoans may be due to artifacts affecting phylogenetic reconstruction (Figure 2).

Four scenarios for the origins of nervous systems. Four scenarios for the origins of nervous systems in the Animal Kingdom depending on the homology of their components and the phylogenetic position of ctenophores. If nervous systems are homologous across metazoans, and if ctenophores are the earliest-diverging animals, then nervous systems were lost in sponges and placozoans. In contrast, if nervous systems are not homologous across animals then they arose more than once, a result that is not made more or less likely by any of the possible placements for ctenophores. Vignettes from phylopic.org and C Nielsen.

The phylogenetic position of Placozoa is also unstable. Some previous studies using mitochondrial markers [17, 18] pointed to an early split of Trichoplax within a monophyletic Diploblastica clade (Porifera, Placozoa, Ctenophora and Cnidaria). However, recent larger datasets dismissed the monophyly of diploblasts and have placed placozoans in different branches, although never as first-splitting metazoan. These positions include Trichoplax as sister to cnidarians [6, 7, 12, 19], sister to bilaterians [14], or sister to cnidarians, ctenophores and bilaterians [16, 20].

Here we highlight potential technical problems related to the placement of ctenophores in the metazoan tree and discuss scenarios of nervous system evolution under different phylogenetic frameworks. We also discuss how studying sponges and placozoans can contribute to our understanding of nervous system evolution.


Fig. 8:

Alternative interpretation of Falcatacaris bastelbergeri n. g. n. sp. in lateral view, inner organs and appendages are simplified, a potential segmental correlation is indicated.

4.3. Alternative interpretation

Clearly, the inner morphology and body organisation of the specimen is difficult to interpret and also allows for different interpretations. Intriguingly, there are a few details in this fossil visible in stereo images (Fig. 4) that trigger interpretations differing from the “simple” one outlined above, but should be still discussed here.

When looking onto the relief of the fossil, the assumption that underneath the rostrum a huge compound eye was positioned is challenged. In this way of documentation it appears that the eye structure is limited to the dorsal part, observable by colouring in the matrix and height difference in the stereo image (Figs. 3, 4). If this is true, the eyes would not be huge compound eyes but rather small and possibly stalked. This would be a more plesiomorphic trait, as we know this character only from strati-graphically older specimens, e.g. from the Silurian ( Haug et al. 2014). It would therefore raise new questions about the phylogeny and evolution of Thylacocephala.

Furthermore, anterior to and slightly ventrally from the gills, there is a ridge running anteriorly across the shield, ending underneath the “stalked eye”, observable only in the stereo image (Fig. 4). This structure could be interpreted as appendage element, and if this is not an artefact of preservation, it could be the “missing” first raptorial appendage, i.e. the maxillula, due to its more anterior position compared to the two raptorial appendages. This in return would make the “third” anterior proximal appendage element indeed a trunk appendage element. This interpretation would also fit better with the overall position of the raptorial appendages. The “third” anterior proximal appendage element seemed to be located too far posteriorly to be of raptorial nature in comparison to other thylacocephalans, in which the raptorial appendages attach closely to each other. This would allow a more “relaxed” correlation of the posterior trunk segments (Fig. 8).

4.4. Comparison to other Jurassic thylacocephalans

The Solnhofen lithographic limestones in the wider sense have so far yielded three distinguishable species of thylacocephalans: Clausocaris lithographica. Mayrocaris bucculata and Dollocaris michelorum ( Polz 1989, 1994, 2001 Haug et al. 2014). These are three of the so far eight known species of thylacocephalans from the Jurassic ( Pinna et al. 1985 Charbonnier et al. 2009 Haug et al. 2014).

When comparing the new fossil to its relatives from the Solnhofen lithographic limestones (C. lithographica. M. buccalata and D. michelorum), one feature clearly separates it from the three known species: its prominent anterior rostrum. This feature is shared with no other Solnhofen representative of Thylacocephala described so far.

In fact, also no other species from the Jurassic shows this feature. All eight species possess more or less distinct rostra, but none of them is comparable to the new specimen with respect to the relative length. There are thylacocephalans with comparatively long rostra – namely Hamaticaris Charbonnier et al., 2017 and Protozoea Dames , 1886 from the Cretaceous (Santonian) of Lebanon, more precisely from Sahel Alma ( Charbonnier et al. 2017). Both additionally stand out from other thylacocephalans by having large posterior spines of a similar length to their rostra. We cannot fully exclude that the specimen at hand once had such a large posterior spine as well, but the structure is not preserved in this specimen. However, posterior spines in thylacocephalans in most cases come with a postero-dorsally running shield outline ventrally of the rostrum base, so that there is a smooth transition from shield to spine. In the new specimen, however, one can observe a distinct shield outline starting from the existing small posterior spine running antero-dorsally (away from a potential large posterior spine). This makes the base of the “potential posterior spine” much narrower and creates a contrast to Hamaticaris and Protozoea. In general, the shield outline in the new specimen is very different from that of Hamaticaris and Protozoea.

Furthermore, the new specimen has a distinct appendage-related notch ventrally of the shield, which is known for other Jurassic thylacocephalans, for example in D. ingens. Yet, they are never developed in a comparable size and extent.

Another feature that distinguishes the new specimen from other Jurassic thylacocephalans is the number of trunk appendages. Most of the better-preserved thylacocephalans appear to have a higher number of trunk appendages, while the new specimen has nine pairs at most. From the Jurassic, only Ostenocaris cypriformis Arduini , 1980 shows eight pairs of trunk appendages ( Pinna et al. 1985).

When comparing the overall shield morphology, the new specimen again differs from other forms from the Solnhofen lithographic limestones. These show rather high shields (longer in dorsal-ventral extent) and a bulged extension centrally of their ventral shield outline. These characters are absent in the new specimen, which shows a comparatively flat shield, almost streamlined in comparison. Species that seem to be more similar in terms of shield morphology are D. ingens (from the Middle Jurassic of France) and Polzia eldoctorensis Hegna et al., 2014 (from the Cretaceous of Mexico). An important distinction, again, is the elongated rostrum, which is known neither in D. ingens, nor in P. eldoctorensis.

A potential explanation for morphological difference in arthropods is often the developmental state of the examined specimen. Therefore, we want to acknowledge this aspect as well. Knowledge on the ontogeny of thylacocephalans is, however, strongly limited. Size differences within the group, and even more importantly, within some species are indeed known to occur. D. ingens for example has been reported with a body size of a few centimetres up to a few decimetres ( Charbonnier et al. 2009). True larval stages, however, tend to differ strongly from their adult stages in many eucrustacean ingroups (e.g., HAUG accepted) and such ontogenetic differences have not been reported for Thylacocephala so far. In addition, no larval stages have been described for Thylacocephala to this point. Furthermore, the specimen at hand does not show any differences in traits with relevance for the functional morphology compared to the record of Thylacocephala. It therefore seems rather unlikely that it is a larval or juvenile stage, but we cannot fully dismiss this theory given that we only have one specimen so far. Of course, if it would turn out that every so far described thylacocephalan representative is in fact a larval form, than we would have to revise this statement.

Sexual dimorphism could also explain the morphological differences of the specimen at hand to its thylacocephalan relatives. But since the sex of thylacocephalans cannot be determined so far ( Charbonner et al. 2017) and there is also no knowledge on their reproductive systems, this criterion cannot be used for interpretation so far. It would first demand for fossil data that enables sex determination and evidence for sexual dimorphism in order to use it as a criterion for interpretation.

The combination of a distinct and long rostrum, the absence of a large posterior spine, a moderately dorso-ventrally compressed body and a distinct appendage-related notch is a unique set of characters among all described thylacocephalan specimens. We therefore interpret the specimen at hand as a representative of a new species of Thylacocephala.

4.5. Systematic palaeontology

Arthropoda sensu stricto sensu Maas et al., 2004 Euarthropoda sensu Waloszek , 1999 Crustacea sensu lato sensu Stein et al., 2008 amend. Haug et al., 2010 Eucrustacea sensu Waloszek , 1999 Thylacocephala Pinna et al., 1982 Falcatacaris gen. nov.

Etymology: Referring to the shape of the rostrum, which resembles a blade weapon of the falcata type.


Watch the video: BIOL 1406 Lecture 01 The Science of Biology Section and (September 2022).


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