lpetrich
December 31, 2003, 12:35 AM
While doing some study of embryonic development, I got an impulse to look for "archenteron" ("primitive gut", what becomes the gut) in PubMed. And I found:
Characterization of Brachyury genes in the dogfish S. canicula and the lamprey L. fluviatilis. Insights into gastrulation in a chondrichthyan. (http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14597203&dopt=Abstract)
Even the abstract was a bit hard for me to follow, but it turns out that sharks and lampreys form their gastrula phase (the act of gastrulation) in a manner very similar to how land vertebrates form theirs (or at least how Xenopus frogs, mice, and chickens do it). This suggests that the big differences in this phase are due to yolk-handling adaptations.
Vertebrate eggs are usually very yolky, and gastrulation often involves wrapping around the yolk, which then becomes topologically inside the archenteron. At least according to these course slides (http://www.zoo.ufl.edu/courses/zoo3713/wolff/FVAslides/Ch4/slidelist.html), amphioxus gastrulation is much like that of invertebrate deuterostomes (hemichordates and echinoderms); some of the embryonic skin infolds, with the blastopore (the resulting opening) becoming the anus. The mouth is formed by the archenteron punching through the embryo's skin on the other side. In vertebrates, however, the archenteron often closes up around the yolk, and a new anus gets formed nearby by the archenteron punching through the embryonic skin.
Interestingly, live-bearing-mammal embryos still grow a yolk sac -- which is still topologically connected to the gut. And form both mouth and anus by this archenteron punch-through mechanism.
But how might the deuterostome gut be related to the protostome gut? Protostomes usually form both the mouth and the anus from their blastopore by dividing that opening in two, in contrast to the deuterostome approach. But if the two guts are homologous, then which openings are homologous? An EMBL team in Heidelberg has investigated that question, examining the expression zones of genes called otx, brachyury, and goosecoid (don't ask me how these names were invented) in a polychaete-larva embryo. Their discovery: (http://www-db.embl-heidelberg.de/jss/emblGroups/g_172.html#summary) protostome mouths are homologous to deuterostome mouths, and likewise for anuses.
But how did these development approaches get started? A common speculation is that the deuterostome one was the original one. Which has some support from the presence of deuterostome-like features in some phyla identified as being in Protostomia by recent molecular-phylogeny techniques, like the lophophorates. For example, see Testing the new animal phylogeny: first use of combined large-subunit and small-subunit rRNA gene sequences to classify the protostomes. (http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11861888&dopt=Abstract)
Possible support for this hypothesis comes from the anatomy of ctenophores (comb jellies). They have a blind gut (one opening for both ways), which runs through most of the length of their bodies. At the aboral (opposite from mouth) end are the apical sense organs; ctenophores swim "backwards", in their aboral direction. So could some early ctenophore-like animal have had a gut that punched through at the (original) aboral end? This location would be near those sense organs, making it appropriate for that opening to become a mouth. The original opening would get the opposite specialization, becoming an anus.
But since I could not find out much on the origin of gastrulation itself, I will stop here.
Characterization of Brachyury genes in the dogfish S. canicula and the lamprey L. fluviatilis. Insights into gastrulation in a chondrichthyan. (http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=14597203&dopt=Abstract)
Even the abstract was a bit hard for me to follow, but it turns out that sharks and lampreys form their gastrula phase (the act of gastrulation) in a manner very similar to how land vertebrates form theirs (or at least how Xenopus frogs, mice, and chickens do it). This suggests that the big differences in this phase are due to yolk-handling adaptations.
Vertebrate eggs are usually very yolky, and gastrulation often involves wrapping around the yolk, which then becomes topologically inside the archenteron. At least according to these course slides (http://www.zoo.ufl.edu/courses/zoo3713/wolff/FVAslides/Ch4/slidelist.html), amphioxus gastrulation is much like that of invertebrate deuterostomes (hemichordates and echinoderms); some of the embryonic skin infolds, with the blastopore (the resulting opening) becoming the anus. The mouth is formed by the archenteron punching through the embryo's skin on the other side. In vertebrates, however, the archenteron often closes up around the yolk, and a new anus gets formed nearby by the archenteron punching through the embryonic skin.
Interestingly, live-bearing-mammal embryos still grow a yolk sac -- which is still topologically connected to the gut. And form both mouth and anus by this archenteron punch-through mechanism.
But how might the deuterostome gut be related to the protostome gut? Protostomes usually form both the mouth and the anus from their blastopore by dividing that opening in two, in contrast to the deuterostome approach. But if the two guts are homologous, then which openings are homologous? An EMBL team in Heidelberg has investigated that question, examining the expression zones of genes called otx, brachyury, and goosecoid (don't ask me how these names were invented) in a polychaete-larva embryo. Their discovery: (http://www-db.embl-heidelberg.de/jss/emblGroups/g_172.html#summary) protostome mouths are homologous to deuterostome mouths, and likewise for anuses.
But how did these development approaches get started? A common speculation is that the deuterostome one was the original one. Which has some support from the presence of deuterostome-like features in some phyla identified as being in Protostomia by recent molecular-phylogeny techniques, like the lophophorates. For example, see Testing the new animal phylogeny: first use of combined large-subunit and small-subunit rRNA gene sequences to classify the protostomes. (http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=11861888&dopt=Abstract)
Possible support for this hypothesis comes from the anatomy of ctenophores (comb jellies). They have a blind gut (one opening for both ways), which runs through most of the length of their bodies. At the aboral (opposite from mouth) end are the apical sense organs; ctenophores swim "backwards", in their aboral direction. So could some early ctenophore-like animal have had a gut that punched through at the (original) aboral end? This location would be near those sense organs, making it appropriate for that opening to become a mouth. The original opening would get the opposite specialization, becoming an anus.
But since I could not find out much on the origin of gastrulation itself, I will stop here.