Biology Reference
In-Depth Information
Lampreys are representative species of agnatha, an ancient group
of vertebrates, while teleosts belong to osteichtyans, actinopterygians
(for their phylogenetic positions, see Fig. 1). There is no report of larval
metamorphosis in any other “fi sh” representatives such as chondrichtyans
(shark and ray) or osteichtyans sarcopterygians (lungfi sh).
This suggests
that larval metamorphosis may have been acquired independently in the
agnathan lineage and the teleost lineage. The opposite role of TH in the
control of larval metamorphosis in lampreys and teleosts further supports
this hypothesis.
Elopomorphs and Pleuronectiforms are representative of some of the
most ancient and most recent subgroups of teleosts, respectively. This
suggests that larval metamorphosis in a biological life cycle has probably
been acquired independently in these two groups during teleost evolution.
The alternative hypothesis is that larval metamorphosis could have been
lost in all the other teleost groups. However, as reviewed in Chapter 5,
transformation of oceanic larva into freshwater juvenile in another teleost
group, the gobiids (order Perciformes), presents typical characteristics of true
metamorphosis, with TH-induced remarkable morphological, physiological,
behavioral, feeding and ecophase changes. We strongly suggest to include
this teleost group among those presenting a larval metamorphosis, in
addition to elopomorphs and pleuronectiforms. As discussed above, this
does not mean that we extend the concept of metamorphosis to any TH-
induced developmental transition.
The thyrotropic axis possesses a major activator role in the control of
larval metamorphosis in representatives of sarcopterygians (amphibians) and
actinopterygians (teleosts: elopomorphs, perciform and pleuronectiformes).
This suggests that the stimulatory action of this axis may have an ancient
origin in a common osteichthyan ancestor to the sarcopterygian and
actinopterygian lineages. An alternative hypothesis is that the stimulatory
involvement of the thyrotropic axis in the control of larval metamorphosis
may have been acquired several times independently during the evolution
of osteichthyans (in amphibians and in different groups of teleosts). As
the thyroid hormones are thought to be inhibitory in the control of larval
metamorphosis in lampreys, a switch in the role of the thyrotropic axis may
have occurred after the emergence of petromyzontidae. The two opposite
roles of THs in vertebrates (and in fi shes) show that, during the evolution
of vertebrates, the role of thyroid hormones in the endocrinology of larval
metamorphosis may have differed dramatically, being inhibitory in lampreys
(representative of a very ancient group of vertebrates, petromyzontidae)
and stimulatory in osteichytans.
