|Oct15-12, 10:58 PM||#18|
How did torsion evolve in snails?
I've been doing more reading and this idea occurred to me:
Many gastropods are hermaphrodites and can self fertilize. Let's stipulate, then, that the common ancestor of all modern gastropods was a hermaphrodite and could self fertilize. A disaster might befall its food supply. Most die. The few survivors have to disperse, to relocate as far from their fellows as possible, in order just to find enough to eat. (This is the situation in which hermaphrodites resort to self-fertilization; there's no one to mate with around.) The lucky ones find enough to eat to reproduce, but only by self fertilization.
How many generations can that go on for without the bad effects of inbreeding? Bad effects, it seems, can happen in one generation:
Even if the ancient gastropods were dispersed for only one generation, that could have been enough to cause the torsion mutation by inbreeding (selfing). When the food supply recovered, the 'damage' would have been done.
In this scenario, torsion would not have been selected because it was advantageous in any way, or even neutral. They are worse off than before, but there was no alternative. Lots of populations survive the disadvantages of a period of inbreeding:
The food supply disaster is an obviously plausible scenario, but it is only meant to stand for whatever force might cause them all to self-fertilize for one or more generations. I can think of others, like the sudden emergence of a gastropod STD: those who mate with others catch it and die, while those who self fertilize are OK. You can invent your own scenario, but forced "selfing" for whatever reason would explain why the disadvantages of torsion became standard in gastropods. Something like this would also explain the apparent suddenness and completeness of the appearance of torsion in the fossil record.
|Oct16-12, 12:18 PM||#19|
The male and female parts of a gastropod body are separated. They don't have little extensions that enable them to pass sperm from one part of the body to the other. I think that gastropods have to a 69 to get pregnant.
I don't think that the torsioning affects the relative position of their sex organs. The sex organs are on the part of the foot that comes after the twist that gives them chirality.
If torsioning did affect their sex act, then I would propose the opposite. Maybe torsioning came about to prevent selfing. However, I don't think the relative position of the sex organs are affected by torsioning.
You may be talking about a type of "founders effect" intensified by "Mendelian segregation." If there were a bunch of snails that had evolved to do selfing, and only selfing, then the different lines of snails would become homozygous for one traite or the other (the segregation). Then, if a mass extinction exterminated all lines except the homozygous "torsioned line", then what would be left are snails that only self and are all torsioned.
The problem is the same. Extant gastropods don't self. There doesn't seem to be a good reason for hermaphrodites to self all the time. There is no advantage to constantly selfing without end.
As you pointed out, selfing is a good thing after a disaster. However, it is a bad thing for most of the time. Furthermore, it doesn't cause mutations.
|Oct16-12, 05:31 PM||#20|
The snails that were forced to self in the study I linked to all developed the same 2 problems: "There was also a significant difference for egg production and juvenile viability over one month; the selfing snails are 94 per cent less fit for these two traits than the outcrossing." I am assuming if we put the offspring of all these snails who had been forced to "self" together, these offspring could resume breeding as normal, fertilizing each other, but that the two bad traits: lower egg production and much poorer juvenile viability, would be expressed in all the successive generations.
The modern pulmonate can self fertilize:
They do this when they happen to end up where there's no one to mate with. But, as the study demonstrates, even one generation of this can bring bad genes to the fore. I'm just asking you to consider a "founder" gastropod which could self: hundreds of millions of years ago; the original gastropod, the only gastropod game in town. (Gastropods which can't self today would have evolved that later, and yet still be descendants of the "founder" population.) Selfing is an advantage because individuals that happen to become isolated can still produce offspring, and the expression of bad traits will most likely be corrected later when there's a rejoining to the original population. The gastropod disaster, however, throws a monkey wrench into that system by forcing all the individuals to self for some brief period, and the result is the offspring all express the "bad" genes for torsion.
Since all gastropods tort, even slugs (who have no shell anymore!) it makes more sense in my mind to think they all got it from a "founder" population which could self and was forced to self, and then couldn't correct the resultant deformity with outbreeding, because there were no non-deformed, non-torted members of its kind left to breed with. It's a disadvantageous deformity. In the millions of years since it first appeared all the gastropods have evolved a myriad of excellent accommodations to it, but they don't seem to be able to get rid of it, just as the Hapsburgs would never get rid of their chin if they kept breeding with other Hapsburgs (or outbreeding with Lenos, for that matter).
The alternative to this way of thinking, as far as I can see, is that there were many, many different kinds of gastropods and they all, at different times, and independently of each other, all found a separate advantageous reason to tort such that now, there are none who don't tort. Seems a stretch. Some tort and then de-tort, but the detorsion is a separately evolved correcting mechanism that came later. If you're a gastropod, you tort (according to wiki). If that doesn't trace back to a common "founder" population, I would think we'd have a lot of gastropods today that just never tort at all.
(Speaking of wiki, it describes the mechanism of torsion:
So, the gene, or genes, for torsion seem to simply instruct the veliger to unilaterally tense up one muscle at a certain time. The article implies, as I read it, that the longer, second stage of asymmetrical tissue growth happens naturally as a result of this constriction (meaning, there's no separate genetic instructions for the differential tissue growth). Do you read it this way?
Regardless, I just googled and found this book:
Which says: "We cannot directly observe torsion on fossils, so it is difficult to demonstrate that any fossil is a gastropod…"
In any event, your original question is a damned good one and I'm really just trying to resolve the cognitive dissonance it produced in me in my clumsy, un-rigorous way.
|Oct16-12, 06:42 PM||#21|
You have cognitive dissonance, too? I myself try not to think about it!
I never thought that one could directly see the torsion in mollusks. However, we can surely see chirality in the shells of mollusks.
Torsion starts with the tightening of a muscle. No fossil there. Torsion would produce chiral shells.
Cephalopods in general have achiral shells while marine gastropods have chiral shells. This rule is true 100% of the time. However, it is a good guide. The reason that few cephalopods are chiral is that they don't torsion.
Cephalopod shells often spiral, but are achiral. Amateur paleontologists (like myself) usually distinguish cepholopod fossils from gastropod fossils by their chirality. It is easy to tell, even for an amateur, which shells have chirality and which don't.
That is what I was hoping some paleontologist would write about chirality. I heard a lecture about mollusks which claimed that there was gradual evolution going from a rostroconch to bivalves and gastropods. Since most bivalves have achiral shells, while most gastropods have chiral shells, I conjectured that the transition from rostroconchs to gastropods showed a stepwise development of chiral shells.
I am sorry I didn't catch the fellow and ask him more questions. That was a fascinating lecture.
|Oct16-12, 07:56 PM||#22|
The reason this is interesting is that pulmonates are also the only order of snails that are achiral as adults. The adult pulmonate is bilaterally symmetrical. This is the group that I told you about before which has to tort twice to be symmetrical as adults.
This leads me to think the first tort is supposed to prevent selfing. If no other snails but the pulmonates are symmetric as adults, and if no other snails then the pulmonates are symmetrical as adults, then it suggests that the chiral asymmetry in other orders may have an advantage in preventing selfing.
Selfing is usually a disadvantage since it brings recessive genes out. So usually organisms evolve in such a way to discourage it. It also has the same disadvantage as asexual reproduction with regards to parasites. An inbred population is usually vulnerable to parasites, which reproduce and evolve fast. Outbreeding allows the population to adapt to new varieties of parasites. So both asexual reproduction and self fertilization are selected against if there are a lot of parasites around.
Let us suppose that the bilateral asymmetry prevents self fertilization.
Since cephalopods are monosexual (do I have the phrase right?), they never had a problem with selfing. Just by being one sex, they avoid self fertilization. So they never evolved a tort at all.
Maybe the first tort evolved among basal snails in an environment which was full of parasites. Self fertilization makes a population vulnerable to parasites. So snails that didn't tort self fertilized themselves, and the descendents tended to die due to parasites. Therefore, the first tort evolved.
Maybe the second tort evolved in an environment where parasites were no so common. Maybe it was hard to find other snails in that environment. Pulmonata are well adapted to dry land. Maybe dry land is more free of parasites. However, it is harder to find snails on dry land. So selfing is an advantage for two reasons.
Therefore, there are two reason to self fertilize. So the second tort evolved to permit the snails to self fertilize. They twisted themselves back in shape again to mate with themselves.
I wish I knew mollusk anatomy better to evaluate this conjecture. However, thank you for starting me on the way.
|Oct18-12, 11:04 AM||#23|
Primitive snails release their sperm in the water. Torsioning can not prevent self fertilization in such animals because the exact position of the sex organs aren’t necessary for self fertilization.
“Most gastropods have separate sexes but some groups (mainly the Heterobranchia) are hermaphroditic. Most hermaphroditic forms do not normally engage in self-fertilization. Basal gastropods release their gametes into the water column where they undergo development; derived gastropods use a penis to copulate or exchange spermatophores and produce eggs surrounded by protective capsules or jelly (see Busycon spiratus photo below).”
Most snails, including pulmonates, have mechanisms to prevent self fertilization. Self fertilization is rare even amound pulmonates.
“Being monoecious organisms, snails have separated the events of sperm transfer and fertilization to prevent self-fertilization. What complicates the organization of their reproductive system is that torsion has resulted in lost structures and other structures are shared by the female and male reproductive systems.”
“Many species are hermaphroditic, but avoid self-fertilization by transferring sperm in packets.”
I found a site which lists some current theories of how torsioning got started. I currently have an interest in the theory that it stabilizes the orientation of larva in a marine environment. Snails that live on land have less reason to torsion. Snails that live on land and have no larval stage, like the pulmonata, have no reason to torsion. Maybe that is why the pulmonata developed the second tort.
“Torsion: The Principal diagnostic criterion for the members of the class gastropoda is torsion. The process of torsion was thought to be due to two different gradual adaptive processes:
(a) To regulate stabilization of the larval equilibrium
(b) To regulate balancing posture in the plantigrade stage. The process of torsion therefore regulates differential growth processes e.g. shifting the mantle cavity into the anterior position. The mantle or shell sinus already existing appears to
be a prerequisite for the survival of such torted animals in not shedding their waste products towards the inhalant currents. The regulative growth also includes the development of the right pallial organs and the right dorsoventral retractor muscle.”
|Oct19-12, 05:53 AM||#24|
The main drift of my thinking has been that it needn't have been an advantage or neutral. It could have represented a definite down grade that, never-the-less, wasn't bad enough to kill them off. The first torted gastropods may, in fact, have had a much poorer quality of life than their immediate and more healthy predecessor, but were able to survive regardless.
The second drift of my thinking is that it was advantageous but for reasons that are lost to history. We would have to know the exact physiology of the first gastropod, and the features of its environment, to understand why torsion was an advantage, things that can't be inferred from modern examples, which have branched out so much that no feature of modern gastropods can be confidently ascribed to the first ancient ones, except torsion.
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