Faculty of Biological Sciences, University of Leeds

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Evolutionary Developmental Biology Lecture 7

 

Multicellularity (OVERHEAD)

Neilson C (1995) Animal Evolution Oxford University Press

 

 

Remember that one of the basic ways of classifying the bodyplans of organisms is related to their cellularity. In most cases we can just count the nuclei: bacteria don't have a nucleus as such, so they can be placed in a separate file. Organisms that do have nuclei will have either one each, or more than one per individual. These are unicellular and mulicellular organisms respectively, and we can open separate files for these as well. The unicellular organisms, or protistans cut across another border, because some of them have chloroplasts, make their own food from sunlight and are clearly plants while other similar forms seem to have lost their chloroplast eat food, are not green and are clearly animals.

 

Multicellularity was obviously a huge step in the formation of the evolutionary bush. How, when and why and how often did multicellularity come about?

 

The earliest eukaryotic organism (i.e. the earliest cell proper) is thought to have existed 1.4 billion years ago. The first multicellular organisms we know about are 800 million years old. This probably doesn't mean that evolving multicellularity took hundreds of millions of years, but that the first metazoa were probably small, soft and didn't make very good fossils.

 

There are a few texts which suggest that the metazoa arose more than one (i.e. are polyphyletic) but most authorities think, with good reason, that they arose only once. The traditional idea is that the ancestral metazoan resembled a choanoflagellate (OVERHEAD). Now what is special about these, and what differentiates them from the many occasions when animals and plants became colonial like Volvox (OVERHEAD)?

 

Volvox is technically a plant. It is seen in ponds as a small green sphere 1-2mm in diameter and made up of around 1,000 individuals, each with two long motile flagella, a nucleus, a light sensitive eyespot and a chloroplast and is embedded in a gelatinous ball. Is this a colony or an individual? The key question is how much co-operation is there between 'cells'? Volvox rolls along through the water as the flagella beat, but one side is always the front. The cells at the front end have larger eyespots, only those at the rear are capable of reproduction. In some species the 'cells' are connected by cytoplasmic strands, but there is no evidence of co-ordination between cells. Each probably responds individually to light and mechanical stimulus.

 

 

In colonial animals the cells, although they may have different shapes and functions, all feed because there is no mechanism for transferring metabolites (e.g. foodstuffs) between cells. You can only make a cell a specialist in sensation, contraction or secretion after you have ensured that it doesn't die of starvation. Choanoflagellates and other multicellular organisms share a series of biochemical and morphological advances which facilitate the condition. A full list would look like this (OVERHEAD).

 

We can summarise this a bit . The rather complex table breaks down into three sorts of characters (OVERHEAD)

  1. Characters directly connected with the function of multicellular organisms, such as cell junctions and complex organic molecules which mediate cell-cell communication like neurotransmitters
  2. Common structural macromolecules/organelles
  3. Structures connected with the sexual life cycle, i.e. haploid gametes, fertilisation and development.

 

We can argue that, if metazoans all contain similar structures they are likely to be related to each other, rather than being the products of multiple protistan/metazoan transformations. Lets look at some common features in metazoans.

 

Cell junctions

 

These are specialised areas of contact between cell membranes. Three main types can be recognised:(OVERHEAD)

  1. occluding junctions which more or less seal off the space between cells so that its composition can be controlled. They will also have an adhesive function. The septate junctions of invertebrates and the tight junctions of vertebrates, tunicates and some arthropods are like this (OVERHEAD). These are seen from the sponges (where they are confined to making a suitable intracellular pond for spicule formation) upwards.
  2. Adhesive junctions, desmosomes (spot or belt) and hemidesmosomes, either cell to cell or cell to extracellular matrix- all metazoan phyla
  3. Gap junctions which allow chemical or electrical contact between cells - hydrazoans and above.

 

Neurotransmitters (OVERHEAD)

These are known from all metazoans, where they are located in synapses. Various RFamides have been found everywhere, acetylcholine in bilateralia only. Acetylcholinesterase,,,, catecholamines and serotonin are found in sponges, but do not seem to have neurotransmitter function.

 

 

Cilia and flagella (OVERHEAD)

 

These terms are used for essentially identical structures consisting of an axoneme of 9+2+2 microtubules occurring in most eukaryotes and perhaps best known in the sperm tail. There is a little confusion because flagellum is also used to describe a much simpler bacterial structure. So lets call them cilia in metazoans.

 

Undulating cilia are used in metazoans to move water away from a cell, or to move the cell through the water with the cell body in front. Most protista swim the other way, following the cilium. The basal structures in choanoflagellates and most metazoans are similar with a basal body and an accessory centriole at right angles to it (OVERHEAD).

Choanoflagellates and metazoa also have flat christae in their flagellar mitochondria while protista have tubular ones.

 

Flagella and sperm

Metazoan sperm are often specialised, but a primitive type, (OVERHEAD) with an ovoid head, an apical acrosome , a body of nuclear material, a midpiece with four spheres of mitochondria and a long cilium with extra centriole at right angles (OVERHEAD) is found in nearly all phyla. In fact I could be describing the human sperm.

 

Diploidy.

Virtually all metazoan cells except eggs and sperm are diploid. There are so few exceptions that it seems likely that the early metazoa were diploid with meiosis preceding the formation of eggs and sperm.

 

Metazoan eggs develop from one of the four products of meiosis, the other three forming polar bodies and degenerating.

Structural proteins (OVERHEAD)

 

Metazoans also have collagen. Similar glycoproteins containing hyroxyproleine and hydroxylysine are found in plants, animals and fungi but those in plants are extensins, and those in fungi are not collagen. Again that makes Volvox an improbable ancestor.

Collagen lives between cells and in basement membranes or more elaborate connective tissues such as our skeleton or an insect cuticle. It is found in sponges, but no one seems to have looked for it in choanoflagellates.

 

Probable monophyletic origin of the metazoa

 

All these similarities point to the fact that metazoans are probably monophyletic, that is all existing and extinct metazoans (with the possible exception of the sponges) arose from non metazoans at the same time, and the nearest surviving thing we have to an ancestor is the choanoflagellates.

 

How do you make a metazoan from a protistan?

 

The next important question is this. Starting from a single eukaryotic cell how do we make a multicellular organism, why would you want to do that, and what are the consequences?

 

The ultimate consequence of multicellularity was of course the heavyweight successful phyla: but that seems to be a bit grandiose for a choanoflagellate to think of, so let's look a little shorter term. (OVERHEAD) One idea is that multicellularity leads to a greater surface area. This is probably quite facile, because most multicellular animals stick together using those desmosomes. But it could be to do with gradients, diffusion, signalling or any sort of transport. Try to think of some ideas for yourselves.

 

Mechanism.

There seem to be three choices.

  • First of all the products of cell division can stay together rather than drift apart.
  • Secondly a number of individuals could come together to make a colony (as slime moulds do at certain points in their life cycle.)
  • Thirdly nuclei could multiply in a mass of cytoplasm (as they do in insect eggs) followed by the laying down of walls.

 

Of these the first is intuitively the best bet, because it mimics a very common developmental process, one cell (the egg) giving rise to a series of genetically identical cells. The original cell, like most eggs, was probably more or less spherical. What conditions are necessary to ensure this? Nurse (in Wolpert 1994) suggests the following

 

1. that the cell grows larger than usual, achieved by a temporary block in mitosis

2. that the mitotic block is released and several divisions follow

3. that the cells stay together.

 

If we assume that something rather like the present cell cycle existed then, the extra growth and mitotic block is no problem. Yeast will grow large in a rich medium and divide at a smaller size in a poor medium. This works by activation of an enzyme p34cd2 kinase. It also establishes an important principle, that the environment controls development in some cases. If you don't like that, or think that I (or rather Nurse) have really been describing the developing egg because that is the way things should be, lets leave most of this out, and just make cells sticky, so that on division they stay together.

 

What happens next? Well we shall get two cells stuck together, and if the next division is in the same plane four and if the next division is in the same plane we shall have made a chocolate orange. This is unsatisfying, because animals don't seem to do that: perhaps the orange segment shape is too long and thin and they need to be rounder - the third cell division seems invariably to be at right angles to the first, making a ball of eight cells. Now eight cells like this make a nice solid ball, but another division makes the cells a funny shape again and the tendency is to make a hollow ball - this would work for anything that wants to be round - try to think of the system with polystyrene balls or oranges instead of cells.

 

Once we have done this we need to impose one further condition: the cells unstick and start again (OVERHEAD). We then have the adult (the multicellular ball) the eggs (the single cells) and embryology (the stages in between). The metazoan has established shared food supply, faster swimming and protection from its large size.

 

Now all cells in this organism, lets call it a blastaea, have to be equal. So there is no pattern. If a pattern arises, that is if cells become different from each other we have to find a reason, a mechanism. Well how about this. Lets suppose that the blastaea lives in a gravitational field, probably the sea. If it is buoyant it will float, if it is slightly buoyant it will sit in the water, if it is heavier than the water it will sink (OVERHEAD).

 

No matter what its overall density we can bet that not all the cytoplasmic contents of the blastaea are equally dense, oils and fats will be lighter than proteins and carbohydrates. In an egg cell, a gamete, they will tend to layer out. The first two cell divisions in nearly all eggs tend to be vertical i.e. aligned with respect to gravity, the next horizontal: cytoplasm will contain more heavy or lighter elements depending on where it sits in the developing embryo.

 

So the egg could be the source of pattern and pass it on to the embryonic cells. Or the blastaea itself could acquire pattern from the environment in another way. If it floats the top will hit air: if it sinks the bottom will hit mud. Air is a discouraging medium for a marine animal, so lets suppose it sinks. The end which hits the mud might stick to it - to form a sedentary adult. Or it might invaginate, because it is stimulated to divide, or it might die, or restrict reproduction to the non-mud end. In any case, an axis has been determined and a pattern set up.

 

We have a slight problem here. Is the signal, whatever it is, institutionalised? We could say that gravity, the signal, follows the ideas of Baldwin and Lamark and becomes incorporated into the genome. This is difficult, and we shall come back to it later. But is it necessary to incorporate it at all? Frogs eggs develop nicely so long as they are in a gravitational field: centrifuge them and they go bananas. Where is the message, in the animal or in the environment?

 

It does seem that in some animals the gravitational signal has been mimicked by an internal signal. and once that signal is there we both no longer need a very precise system of cell divisions to generate an embryo and at the same time have the means to do it another way. So we have two polarised lots of animals, one of which has very precise cell divisions achieved with reference to outside forces or signals - mosaic development - and another which seems to make a bunch of cells and then tell them what to do - regulatory development. Most animals are somewhere in between. One problem which we have not solved is that the ones with precise cleavage patterns are usually very good at regeneration: and one would think that regeneration needed precise signalling about what is missing before a new bit can be made.

 

The organism we have made, the blastaea, is radially symmetrical. Bilateral symmetry seems to be an early metazoan characteristic, and requires the specification of another axis (OVERHEAD). In sexually reproducing animals like frogs this is often defined by the point of entry of the sperm. But asexually reproducing animals can be bilaterally symmetrical too - so we need another candidate and don't have one at present.

 

This separation of the specification of differences from the origin of a cell by means of a signal gives us the concept of positional information. A cell in a mosaic embryo has positional information by virtue of its position: I am unique (or one of a small group) I know I am 3/8 of the way down the vertical axis because I have a light particle value of x and a heavy particle value of y (if that is the mechanism). Because of these parameters I shall grow without dividing and become an egg (OVERHEAD) A cell in a regulating embryo has positional information because it is told: it receives a signal from another cell. Almost all animals have genetic information which specifies position along an axis e.g. homeobox genes which we shall consider in due course.

 

Francis Crick remarked that embryos seem to be very fond of stripes. By this he meant that periodicity was very common along our specified axes. This will allow segments in vertebrates, arthropods and worms or even tentacles in hydroids and jellyfish. The striping mechanism may have evolved several times, because it differs in Drosophila and leeches, and in things like hair follicle or feather spacing in mammals and birds. But it is a recurring theme.

 


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