Register | Sign In


Understanding through Discussion


EvC Forum active members: 65 (9162 total)
8 online now:
Newest Member: popoi
Post Volume: Total: 915,814 Year: 3,071/9,624 Month: 916/1,588 Week: 99/223 Day: 10/17 Hour: 6/1


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
Author Topic:   Descent with Modification v. Larval Hybridization
Blue Jay
Member (Idle past 2697 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 7 of 23 (479146)
08-24-2008 9:06 PM
Reply to: Message 4 by Fosdick
08-24-2008 8:07 PM


Re: Trochophore larvae
Hi, Hoot Mon.
I won't outrightly disagree with this hypothesis, because it seems like it could be interesting if any evidence is found for higher eukaryote HGT.
However, I do notice a couple of problems. Your quote from Williamson and Vickers:
quote:
Further hybridization with rotifers gave trochophore larvae to the ancestor's of today's clam-like and snail-like mollusks. Their close relatives, the octopuses and squid, lack larvae. In conventional thinking, larval forms arose over time as young and adult forms within a species became more and more different. The similarities among larvae in distantly related species are conventionally explained by convergent evolution”many organisms developing larval stages to solve problems such as dispersal and feeding.
I think the lack of the trochophore larva in cephalopods is more easily explained by the loss of the stage, while clams and snails retained it. The image from your message is provided below for a reference. Cephalopods are most often considered a derived branch or sister-group of the gastropods. The loss of a larval stage isn't unpredecented, either: lots of frogs skip the tadpole stage, and lots of groups of snails don't have trochophores, either.
Also, as far as I can tell, the animals with trochophore larvae are still considered to form a monophyletic group (along with various groups wherein the trochophore was apparently lost). I don't believe it actually is hypothesized that the trochophore was developed convergently in these lineages (though I'll admit to a layman's expertise, at best, on this topic): most evidence seems to suggest that the trochophore was a plausible ancestral condition, and all non-trochophores in the clade are the derived condition.
Edited by Bluejay, : dBCodes problem

-Bluejay
Darwin loves you.

This message is a reply to:
 Message 4 by Fosdick, posted 08-24-2008 8:07 PM Fosdick has replied

Replies to this message:
 Message 12 by Fosdick, posted 08-25-2008 11:52 AM Blue Jay has replied

  
Blue Jay
Member (Idle past 2697 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 8 of 23 (479147)
08-24-2008 9:14 PM
Reply to: Message 5 by gluadys
08-24-2008 8:22 PM


Re: Trochophore larvae
Hi, Gluadys. And, welcome to EvC!
gluadys writes:
As you say, it would still involve descent with modification to proceed from the initial hybridization, so there is still time needed for the transformation of the species.
I disagree with you on this. Assuming he's right that HGT can and, in fact, did happen in eukaryotes, isn't it entirely possible that a new species is formed immediately following an HGT event? Of course it would be difficult for this to happen in a bisexual species unless that specific HGT event was unusually common. But, snails are hermaphroditic, which always permits the possibility of self-fertilization (though I don't know how much of this is actually allowed by gastropods).
But, speaking purely theoretically, Hoot Mon could be right that larval hybridization would not require an extended period of descent with modification.

-Bluejay
Darwin loves you.

This message is a reply to:
 Message 5 by gluadys, posted 08-24-2008 8:22 PM gluadys has replied

Replies to this message:
 Message 10 by gluadys, posted 08-24-2008 10:15 PM Blue Jay has not replied

  
Blue Jay
Member (Idle past 2697 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 15 of 23 (479293)
08-26-2008 8:57 AM
Reply to: Message 12 by Fosdick
08-25-2008 11:52 AM


Re: Trochophore larvae
Hi, Hoot Mon
Hoot Mon writes:
But trochophore larvae are not monophyletic (again, per Williamson & Vickers' diagram):
I have always read that all the phyla in that diagram (Sipuncula, Mollusca, Annelida and Rotifera) are grouped in the clade Lophotrochozoa, whose common ancestor was proposed to have had a trochophore larva.
I realize that there is some disagreement as to the placement of the phylum Rotifera, which compounds the matter significantly. But, I don't know enough about this subject to make an informed conclusion.
I remain skeptical of this larval-hybridization hypothesis, simply because the two alternatives (monophyly and convergence) are apparently at least as robust as it is. It could be tested, I think: you could sequence genes that are active during larval development, and separately sequence adult genes, and plot separate cladograms based on adult and larval genes. If the larval cladogram resolves a trochophore clade while the adult cladogram does not, this could support Williamson's hypothesis. Granted, the process would be a pain in the butt, but the technology exists (I think).
If Williamson's hypothesis turned out to be correct, it would be a very interesting development in evolutionary biology. At this point, it seems to be a violation of parsimony to me, but I'll withhold final judgment until it has been tested.

-Bluejay
Darwin loves you.

This message is a reply to:
 Message 12 by Fosdick, posted 08-25-2008 11:52 AM Fosdick has replied

Replies to this message:
 Message 16 by Dr Jack, posted 08-26-2008 12:32 PM Blue Jay has replied
 Message 17 by Fosdick, posted 08-26-2008 3:19 PM Blue Jay has replied

  
Blue Jay
Member (Idle past 2697 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 20 of 23 (479385)
08-26-2008 6:19 PM
Reply to: Message 16 by Dr Jack
08-26-2008 12:32 PM


Re: Trochophore larvae
Hi, Mr Jack.
Mr Jack writes:
if this genetic transfer is occuring why aren't we seeing evidence of it on cladistic trees reconstructed from genetic data?
I would think it's for multiple reasons:
  1. Most cladograms are based on just one or two genes, which are not chosen for their roles in development.
  2. Cladists use the same handful of genes for all cladograms they make.
  3. There is less funding for research into invertebrate taxonomy than for, say, human cancer research or grizzly bear monitoring.
  4. The majority of people don't give a wet slap about worms and slugs.
Now, I can't, for certain, say whether these are all in effect over this particular issue, but it seems likely to me. Genes used in cladograms are selected for their estimated mutation rate, not for their activity level at different periods of development. And, since larval development is often quite ephemeral and larvae are often smaller and harder to find and process, scientists probably try to stick with adult-active genes.

-Bluejay
Darwin loves you.

This message is a reply to:
 Message 16 by Dr Jack, posted 08-26-2008 12:32 PM Dr Jack has not replied

  
Blue Jay
Member (Idle past 2697 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 21 of 23 (479386)
08-26-2008 6:25 PM
Reply to: Message 17 by Fosdick
08-26-2008 3:19 PM


Re: Trochophore larvae
Hi, Hoot Mon.
Hoot Mon writes:
the trochophore larvae can be found in both protostomes phyla (e.g. annelids) and deuterostomes phyla (e.g., echinoderms)
Really? Are you sure it's in echinoderms? I've never heard that before. Williamson and Vickers say it's in echiurans, but those are lophotrochozoan worms.
Can you find me a source for this? If you're right, it would lend at least a little credence to this concept, I think. However, I'm still not sure how it would compare to convergent evolution in terms of parsimony and evidence.
You'll still have to do the cladistics test I proposed before I'm fully convinced.

-Bluejay
Darwin loves you.

This message is a reply to:
 Message 17 by Fosdick, posted 08-26-2008 3:19 PM Fosdick has replied

Replies to this message:
 Message 22 by Fosdick, posted 08-26-2008 7:38 PM Blue Jay has not replied

  
Newer Topic | Older Topic
Jump to:


Copyright 2001-2023 by EvC Forum, All Rights Reserved

™ Version 4.2
Innovative software from Qwixotic © 2024