Faculty of Biological Sciences, University of Leeds

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Evolutionary Developmental Biology Lecture 8

 

Introducing Larval forms (OVERHEAD)

 

 

Last time we looked at the first metazoans and the way in which we think they initially formed a hollow ball of cells with a cavity inside: we called these hypothetical organisms blastaea, looking rather like a colony of choanoflagellates (OVERHEAD) but probably with more cellular specialisation. This animal has a nice isolated cavity, the blastocoel cut off from the outside world and perhaps serving as a means of passage for organic molecules from cell to cell without getting lost. This will allow cells which don't feed to survive.

 

We suggested that the organisation of this animal might be affected by its environment - that it might sink to the bottom, and that contact with the mud might set off a cellular reaction leading to the defining of a ventral pole - a bottom end. Lets look now at an alternative scenario: many organisms live in water rather than ion the mud. How might a free swimming blasteae cope better with swimming in the sea?

 

A blastaea would roll through the water with no special preference (OVERHEAD): probably the first thing to happen was a preferred direction of swimming, with sensory cells collected at one particular place - lets call it the apical organ which goes first.

 

The side opposite the apical organ was now to leeward and offered the opportunity of retaining food particles which didn't get brushed aside as the organism rolled through the water. In this position we might find digestive cells. Localised cell division would perhaps be required to ensure an adequate food supply. Local multiplication of cells in this area would make it larger, and it is possible that to fit in more cells the animal formed a dent with the new cells pushing upwards into the hole in the middle of the ball of cells and we might even find an inpushing, a cavity, a gut lined with digestive and absorbing cells. All cells would retain a cilium, which would be used for locomotion or for food transport.

 

 

Now if we simply flip this over through 180o we find that we have made a cnidarian coelenterate (OVERHEAD) radially symmetrical, with the opening of the gut pointing upwards and the apical organ stuck in the mud - no great loss because it isn't going anywhere. This would again be caused by a change of lifestyle, a specialisation to a potentially better environment, and we already have two possible points at which an animal may have settled on the bottom, either as a blastaea, or as a specialisation of a free living planktonic (swimming in the sea) form.

 

The size of both hydroids and planktonic animals was, and still is, limited by the effectiveness of the one cilium per cell set-up. Cilia are small, and have a limited power output.

 

The next advance we see, in the plankton appears to be the formation of a band of compound cilia, which are more effective. Compound cilia (more than 1 per cell), have greater power output.

 

The story isn't quite as simple as it might be, however, because this sort of modification seems to have arisen twice, and the compound cilia function differently in each case (OVERHEAD). Ciliary bands (which is how they usually occur) are classified according to function as upstream or downstream. A downstream band would give better swimming, allow an increase in body size and waft food particles towards the gut opening.

 

So lets modify our planktonic organism (OVERHEAD AGAIN). The downstream ciliary collecting band or archaeotroch naturally goes around the opening of the gut.

 

If we believe in an animal like that we can fill in a few more probabilities too, an apical sensory organ as before, a ring nerve below the archaeotroch and thus around the mouth and probably several links (because this animal is radially symmetrical) between this ring nerve and the apical sensory organ.

 

The least efficient feature of this animal is now the one way gut. Let us suppose that a series of secondary openings, gastrial pores, appeared around the apical organ. Since the beast is radially symmetrical the minimum number of such pores is likely to be four (two implies bilaterality, three implies a funny symmetry that we don't see, and we do find four outlets from the gut in some coelenterates). These would initially serve as outlets for indigestible particles i.e. anuses.

 

Perhaps the biggest leap comes next, and we can choose what to believe to a certain extent. It seems likely that only one of the four gastrial pores was retained. The other three might have become hydropores (hydrostatic skeletons, gills, coelom?) or become disused. Just why this happened is a mystery, and we will come back to that point later. We know that it did happen because we find, in the modern plankton, animals which look very like our theoretical animal (OVERHEAD). In fact they are the trochophore larvae of several modern groups of marine animals. They have the required apical organ, the required hole at the other end acting as an anus and a single mouth further up the body, i.e. a through gut. So far this fits well with our ideas. But unfortunately the ciliary band lets us down. In these animals the downstream collecting band (the old archaeotroch) is represented by two areas, the telotroch around the anus and the prototroch around the mouth. What has happened is fairly clear, because neither of these bands is complete and a region of primitive, one per cell cilia joins the two. It seems probable that the protostome larva arrived at this stage not by a second mouth appearing but by a splitting of the original gut opening into two, both retaining part of the archaeotroch.

 

Also swimming about in the sea are superficially similar larvae (OVERHEAD) with many of the same features: an apical organ, a ventral anus, a single more dorsal mouth. These however have an important difference (OVERHEAD) in that the neotroch, the ciliary band around the mouth is upstream, not downstream. Surely these are a better candidate for our earlier story of a second opening for the gut breaking out towards the apical organ.

 

If we accept that, of course we have to account for a different set of circumstances which might have led to the split of the original opening in protostomes. How about this. The protostome is suited for a pelagic life, swimming in the sea. We can speculate some more and suggest that it might have taken advantage of a niche and settled down on the bottom to take advantage of a particularly succulent piece of mud (OVERHEAD). Once settled on a surface consequences follow. First of all a preferred direction of travel across the surface might be established giving rise to a front end. If there was a preferred front end then the animal might elongate a bit along its new axis, and food might start to move in a different way in the archenteron, in towards the front, out towards the back. This would be more efficient. Elongation of the mouth of the archenteron might lead to collapse in the other plane, and the 'sides' of the archenteron coming together. The archaeotroch would probably be redundant in a bottom liver and might be lost or reduced. Further small modifications might include the permanent fusion of the lips of the blastopore and the modification of the nervous system to a ring round mouth and anus connected by a double ventral nerve cord. The nerves associated with the apical sense organ would become the brain.

 

The mouth and anus are thus both formed from the entrance to the archenteron and the archaeotroch is split into two, an incomplete ring of cilia around both orifices. Around mouth anus and along the fused lips of the archenteron run cells with single cilia. There is an apical organ, and in forms that settle down a brain and a paired ventral nerve cord. We would have to speculate that at some point this revolutionary approach was discarded and the animal became pelagic again. But this need not be permanent: the trochophore larva forms an early stage in the life cycle of many animals which live on the bottom mud as adults.

 

Confusingly if the deuterostome type where the band of cilia around the anus is a downstream archaeotroch and is still complete and the cilia around the mouth form an upstream neotroch settled down on the sea bottom (OVERHEAD) we would finish up with a very similar sort of animal. The brain is still formed from the apical organ, but is connected to an unpaired nerve cord, which, in sedentary forms happens to be dorsal.

 

Why am I telling you all this? Because, if we leave aside the coelenterates and look at bilaterally symmetrical animals we find that they all look fundamentally similar (OVERHEAD) , usually with a brain, sensory organs and ventral nerve chords (but sometimes dorsal like vertebrates). But if we look at them more carefully, and also take into account their embryology and larvae we find that they fall into two main groups, protostomes and deuterostomes. In the first group the protostomes, the blastopore becomes divided into adult mouth and anus by lateral fusion of the blastopore lips, the right sort of interrupted cilial bands and nervous system are present. The same, broadly goes for the deuterostomes, where the primary blastopore becomes the anus and the mouth is secondary. We find, in fact, most animals are protostomes, and just the echinoderms and chordates are deuterostomes (OVERHEAD).

 

Now, I have partly deliberately led you into an enormous mess. I was talking about evolution and primitive metazoans and I have gradually switched to embryology. This was the same confusion that overtook embryologists when comparative zoology first became popular. It came as a blinding revelation to the nineteenth century biologist that evolution and development were really the same thing. Or that they were similar. Or something. So we need to examine this confusing idea more closely. Just what is the relationship between embryology and evolution? Can we really learn about evolution by studying embryology? Well, to some extent, yes.

 

Why protostomes and deuterostomes? This raises an important point. We can guess that the two types arose independently by chance. We can say that one type is primitive and the other a derivative. We cannot say that the first hole mouth, second anus arose by a mechanism such as the water flow or distension pushing the closed end of the gut up against the outer layer predisposing a hole. Why not? Because the embryo doesn't feed any more. It lives on yolk. We have to suggest, if we want this sort of mechanism, that what is now the larva, or the embryo, was once an adult where this sort of mechanism could, with some difficulty, be incorporated into the genome.

 

Once its in the genome the same developmental stages will happen whether or not the initial causative factor is present i.e. selection is no longer an issue. There are many things that an embryo doesn't do - many things don't have to work at this stage. Selection for a through gut will work on the adult, which is more efficient, but how the gut was formed is not an issue: there are no better nor worse ways, just different ways.

 

Mutation, chromosomal change etc. will produce embryos that do things differently, and the adults produced will be subjected to selection. This supposed smallish changes and an embryonic change distributed through a population: big changes, hopeful monsters, would probably never meet.

 

Animals with primitive cilial bands exist as larval forms. The idea of a larval form is in itself interesting. Embryos normally live on their fat, usually in the form of yolk. Yet annelids, molluscs and flatworms share a larval form which feeds, and which all look very similar. A little later, after a period of feeding these larvae undergo complex metamorphoses into very different adults. So where does this fit in? We could suggest that the larva is a primitive form, common to all these types and from which they evolved. But how?

 

How could a free swimming trochophore evolve the ability to metamorphose into a set of different adults? If common sense prevails we see the larva as an interruption of development, perhaps as a necessity for feeding or dispersal: the tadpole is clearly a modification of the early embryo which feeds and swims: tadpoles do not evolve legs, they develop them. Next time we will look a little more deeply into this distinction.


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