Plant evolution ...help!!?

Question

A major change that occurred in the evolution of plants from their algal ancestors was the origin of a multicellular diploid stage. What advantage would multicellularity provide in this stage of the life cycle?
a.enhanced potential for independence of the diploid stage from the haploid stage
b.increased fertilization rate
c.increased gamete production
d.increased size of the diploid stage
e.increased spore production from each fertilization event

Answer 1

Wow! Usually multiple choice questions are incredibly stupid (giving one obvious choice, and several easily eliminated, or even absurd 'alternatives') ... and they usually teach the student almost nothing ... especially if the student just comes here and asks what letter to circle. (So I'm assuming that you've thought about it a bit and don't know how to approach the question.)

But this one is incredibly subtle and takes some thought. So I think the answer is (e) ... but please read this carefully ... don't just circle the answer ... and understand why this is a *tough* (and advanced) question.

First, let's clarify that this is not saying that multicellularity and diploidy evolved *together*. Evolutionary theory holds that the evolution of diploidy (and thus sexual reproduction) occurred *long* before multicellularity.

So the question (as I understand it) is this ... given that an organism already has alternating haploid and diploid stages in its life cycle (e.g. green algae) ... what advantage to reproduction itself is there for the diploid stage to become multicellular.

For the answer, look at a haploid-diploid, but single-celled organism that reproduces by meiosis (a diploid individual splitting into two haploid individuals) followed by fertilization (or fusion) of two haploid cells into a single new diploid individual. If such an organism is reproducing away (expending a lot of energy), in a short time it is a little ball of reproducing cells. But in the interior of the ball, the cells are going through the trouble of meiosis, splitting into two haploid cells, and then immediately recombining with other identical cells. So this defeats the entire advantage of sexual reproduction ... the recombination (or cross-fertilization) with other genetically different individuals to produce more genetic variation. It is only at the outer edges of this ball that the haploid-stage cells can encounter haploid-stage cells from another individual.

Multicellularity improves this situation dramatically. The interior cells can undergo mitosis (not meiosis), where the diploid cells simply replicate genetically identical copies of themselves ... which requires far less wasted energy than meiosis. This allows the organism to grow in size, and even to have dispersal organs that shoot haploid cells (spores) out into the world far away from original organism, thus increasing the chances of fertilization (fusion with haploid cells from another individual). The only time meiosis occurs is specifically to produce those haploid cells (spores or gametes) used for reproduction.

So let's consider our candidates:

(a) Yes, it does raise the potential for independence between diploid and haploid stage ... but *by itself* this doesn't qualify as an "advantage";

(b) Hmm ... very tricky! It increases the *chances* of *fertilization* (recombination of haploid cells from two genetically different individuals) ... but not the *rate* of fertilization (as it takes a plant a lot longer to grow to sexual maturity to be able to disperse spores or seeds than algae);

(c) No, this has no advantage or disadvantage to gamete production;

(d) Yes, it increases the size of the diploid stage ... but again, *by itself* this doesn't qualify as an "advantage."

(e) Yes ... the reason it is an advantage to increase spore production for each fertilization event is to increase the chances of *more* fertilization events ... which ultimately is the *primary* advantage of sexual reproduction as far as natural selection goes.

So I think the answer is (e).

Answer 2

Seems to be kind of a forest through the trees question. The simple answer is that multicellular organisms have the ability to have each cell specialize in a life cycle function. Therefore some cells can focus just on photosynthesis and others can focus just on reproduction, making the whole organism "more productive"