luvluv

Okay, I was following this thread up until the time lptreich started typing.

Hey folks, I went to an arts school.

Type slow.

KeithHarwood

I was under the impression (from first year Biology many years ago) that for many primitive plants the individuals were the haploid phase and the diploid phase was a single reproductive cell. (Primitive, in this case meaning moulds and mosses and such-like.)

Doubting Didymus

Actually 'primitive' plants and algae generally have two complete adult phases, one diploid and one haploid. Each one produces the other, (the diploids produce haploids by meiosis, making spores, and the haploids produce diploids by having sex, sending out gametes)

You may be thinking of fungi, where the nuclei are all haploid, and each individual fungus has 'male' and 'female' nuclei in its body. 'sex' in a fungus is when two of these nuclei fuse together (karyogamy), producing a diploid cell. This cell instantly undergoes meiosis to form spores, so there is only a very breif diploid period in a fungus.

Luvluv, is there anything, particularly from my earlier post, that you would like me to explain more simplistically? I would be happy to oblige.

luvluv

Well, firstly doubting, aren't meiosis and mitosis EXTRAORDINARILY different processes? That seems a rather large evolutionary mutation for a single organism to take, much less a few of them in the same area.

But, more than that, I guess my real question was about the first animal that was essentially male and the first animal that was essentiall female. That is to say, the first animal that truly had a gender and could not reproduce with another animal of it's own gender.

(I really shouldn't be allowed to spell without adult supervision)

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Doubting Didymus

No, not at all. Meiosis is just exactly the same thing as mitosis, really. the only difference is that the two new cells don't stop the division process, so they keep going until there are four cells with half a genome. It really is an extremely similar process, and to my mind at least it just screams 'mutation error!'

So no, it would not be a very big step at all, and there is no problem with more than one organism doing it at the same time in the same area, because the chances are that the mutation that caused meiosis would be a tendancy, not a definite. So, unicelluar organisms being what they are, a population of asexually produced organisms could exist, all with the tendancy to undergo meiosis occasionally.

There would not have been any first male or female organism, what would gradually occur is for different groups of a single populations gametes would become specialised. This would happen for a very good reason, particularly in multicellular organisms.

A hypothetical:
a simple multicellular organism requires that its offspring be nourished during development. Also, the eukaryotes need at least one of their gametes to contain all the little organelles, so the sensible solution is to have a large gamete, which can contain all the organelles to pass on to the child organism and can contain a small 'booster' of energy and nutrients for it to get started.

However, the large gamete has a problem: it's movement is hindered, because it is so large. There is therefore another selection pressure in favour of small, mobile gametes.

This situation sets the scene perfectly for an individual with a small mutation, causing it to produce some gametes that are smaller, and some that are larger. This organism takes advantage of the best of both worlds. Eventually, selection pressure would favour organisms with both really massive gametes, and really really small and fast gametes.

The population of organisms that we would be seeing would be producing what we now know as sperm and egg, on the same individual. This is how the gametes became 'male' or 'female' gametes(much of life never went any further, so this is actually the norm for most living things today).

Your question, however, is how organisms that are solely of one gender evolved. Well, it would start with an organism producing both sperm and eggs. This organism has a problem: it breeds with itself too often. When an organism breeds with itself, the offspring are not very variable. There is therefore a selection pressure in favour of organisms that prevent their own self - fertilisation.

Various organisms go about this in different ways. Most flowers have their female parts become fertile after their pollen is dispersed, for example. A different solution, however, would be to have the male gametes only come from one organism, and isolate the female parts on a second organism. This could happen very gradually, with genes for being more male than female, as well as the opposite genes for being more female than male, becoming graually more and more prominent until eventually the population has no hermaphrodite individuals.

To summarise:

1) organisms benifit from having some larger and some smaller gametes.
2) after a time, the difference in size becomes so pronounced that we would call the different gametes 'sperm' and 'eggs'
3) both kinds of gametes would be produced on the same individual, so the organism frequently fertilises itself.
4) a population that has individuals that lean towards either egg production or sperm production would have more cross- fertilisation and less self - fertilisation, giving that population anadvantage in terms of the variability of its offspring.
5) this tendancy towards one gender or the other would eventually result in organisms that are entirely of one gender or the other.

So becoming gendered was a gradual transition, and both genders evolved slowly and simultaneously. There would never have been a male that had no female to mate with.

luvluv

Okay I get that, but how would these two complete organisims with only half a genome a)survive (wouldn't it just shrivel up and die without all of its genetic information?) and b) what mechanism would the genetic information in one of the cells have for combining with the genetic information in the other cell. I mean, there's a cell wall and all that other stuff in the way.

Is there like a book or something on this subject I can get? It might be easier than me repeatedly asking you questions that may very well be stupid.

luvluv

Okay, what about this:

"There are several important differences between mitosis and meiosis.

First, mitosis results in the formation of two daughter cells identical to the original cell. Meiosis forms four gametes: cells with half the chromosomes of the original cell.

Second, mitosis is a single cell division. Meiosis is a replication followed by a reduction.

Third, mitosis has four stages. Meiosis has eight stages.

Fourth, in prophase of mitosis, the individual chromosomes line up in the middle of the cell and the chromatids are pulled to the poles. In prophase I of meiosis, the tetrads line up in the middle of the cell and the chromosomes are pulled to the poles.

Fifth, there is no crossing-over occuring in mitosis. In meiosis, crossing-over occurs in prophase I."

It's the crossing over that's the tricky bit.

Doubting Didymus

Most introductory biology texts would cover the basics, but I think there are some books especially on the evolution of sex. I'll get back to you.

For now, I don't at all mind answering your questions.

a) Actually a haploid cell does have a full working copy of the genetic information. It's the diploid cell that has two copies, I am not sure what for. Some organisms in fact are made entirely of haploid cells of this kind. You are, however, correct in assuming that a unicellular organism could probably not survive with only half its genes.

b) there are a few mecanisms for this, the most prominent of which is 'conjugation', a kind of sexual gene - transfer mechanism employed by paramecium and some others. Cell walls aren't all that sturdy, and at the unicellular level there is a blurred line between eating something and combining with it.

Doubting Didymus

Yes, modern meiosis and modern mitosis are fairly different, but they have been evolving for millions of years, and the early form of mitosiscould have been very different, and much more similar to mitosis.

Peez

If there were no important differences, we probably would not have separate names for them. There certainly are important differences, but there are more similarities than differences. These three are not independant differences. Meiosis results in the formation of four haploid cells because it involves two cell divisions, each with four phases. As an aside, some biologists recognize five phases, including prometaphase. Either you have misquoted, or your source is in error: chromosomes line up in the middle of the cell during metaphase, not prophase. This is the most critical difference between mitosis and meiosis, the part that results in two haploid cells being produced from one diploid cell. (Again, crossing over takes place during metaphase I.) Crossing over can only occur because of the way that homologous chromosomes line up in metaphase I, so this is just due to the way that chromosomes line up in metaphase I of meiosis.

Thus, there are really just two fundamental differences between mitosis and meiosis:

Mitosis involves one cell division while meiosis involves two.

In mitosis chromosomes always line up individually while in meiosis I (but not meiosis II) pairs of homologous chromosomes line up.

The other differences flow from these fundamental differences (diploid from diploid vs haploid from diploid, four phases vs eight phases, no crossing over vs crossing over, etc.). Note that crossing over is not required for meiosis to work, and may have evolved later.

Meanwhile, in what way are they similar? Both start with a cell that has duplicated its DNA (it would be easier for meiosis to start with a cell that has not duplicated its DNA, and so would not need to divide twice), both divide the nucleus before dividing the cell (strictly speaking, mitosis and meiosis refer to the division of the nucleus, not the cell), both start with the chromosomes condensing while the nuceolus disappears and the nuclear membrane breaks up (prophase), both then involve the lining up of chromosomes in the middle of the cell guided by the spindle between the centrosomes (individually in metaphase of mitosis and metaphase II of meiosis, pairs in metaphase I of meiosis), both then pull half the chromosomes to either end of the cell (anaphase), and in both the chromosomes then disperse in the reforming nuclear envelope at either end of the cell (telophase), followed by cell division (cytokinesis).

So, despite the fact that there are (very) important differences between these two types of cell division, they are actually very similar. It is not difficult to see that meiosis evolved from mitosis.

Peez

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