Convergence as a Function of Preadaptation

Let’s build on the convergence between vertebrates and cephalopods.  This time, let me quote some excerpts from the following article: Squid vascular endothelial growth factor receptor: a shared molecular signature in the convergent evolution of closed circulatory systems by Masa-aki Yoshida, Shuichi Shigeno, Kazuhiko Tsuneki, and Hidetaka Furuyaa (in EVOLUTION & DEVELOPMENT 12:1, 25–33 (2010)).

First, the researchers lay the groundwork for the convergence of these two systems:

Metazoan animals have evolved an incredible diversity of hearts and heart-like structures. The most elaborate case in invertebrates is observed in coleoid cephalopods: they exhibit an elaborate closed circulatory system (Schipp 1987; Budelmann et al. 1997). Their heart possesses a kind of advanced output structure similar to that of the human heart, which differs largely from molluscan typical nonendothelium primitive chambered hearts (see Kling and Schipp 1987; Schipp 1987). Neither morphological nor molecular data give strong support to a close phylogenetic relationship between vertebrates and cephalopods, suggesting that the closed circulatory systems and complicated hearts were formed independently in each lineage, and have converged during their evolution.

and

Their cardiovascular system is considerably similar to vertebrates in several respects such as high oxygen binding capacity, high concentrations of proteins, and short circulation time (Schipp 1987). Each vessel in the cephalopod is constructed similarly to vertebrate vessels, with an endothelial lining on a basement membrane (Budelmann et al. 1997), although the cephalopod blood vessel lining does not have the cellular junction typical among vertebrate species. Most invertebrates have no endothelium in their vascular walls so the cephalopods are unusual in that they are invertebrates with vertebrate type blood vessels. As the other molluscs have open vascular system, the peculiar blood vessel configuration in the cephalopods is in all probability secondarily developed similarly to the vertebrate among chordates (Ruppert and Carle 1983).

Then, they find something really cool.

In this study, we discovered a VEGF receptor ortholog in squid and identified the expression in blood vessels of embryos.

Whoa.  VEGF receptors are receptor tyrosine kinases devoted to the development of blood vessels.  The squid have independently recruited the same protein for the same purpose!

This is the first evidence to show commonality in signaling molecules between closed circulatory systems in the cephalopods and vertebrates. We also found that domains in developing cardiovascular systems of squid embryo express the VEGFR, together with the FGFR. In addition to the finding of conserved patterns of transcription factor Nkx2.5/Csx gene in cardiac progenitor cells (Elliott et al. 2006), these conserved genes play roles as part of developmental pathway in heart development. Thus, we propose that the conserved molecular developmental program for cardiovascular systems were recruited independently to the closed circularity systems of cephalopods and vertebrates.

and

We found a squid VEGFR ortholog that is expressed in the developing blood vessels, notably in the sheet-like endothelial cells of the systemic and branchial hearts. The highly restricted localization of VEGFR in the vascular endothelial cells and its shared expression pattern in the developing hearts of cephalopods and vertebrates suggest a shared molecular signature of closed circulatory systems that has been independently elaborated during evolution.

In fact, there is other evidence that suggests this receptor existed very early in metazoan evolution and was devoted to the formation of the heart:

The expression of VEGFRs is commonly observed in the endothelial and endocardial cells of vertebrates, as well as endothelial cells of cephalopods. The vertebrate endothelium is similar to their endocardial cells with respect to having junctional complexes including VE-cadherin (Larson et al.2004). Both types of cells are known to be derived from a multipotent progenitor by the presence of a common transcriptional network (Ferdous et al. 2009). Taken together, we propose (a) that the VEGF pathway may have been involved in cardiac formation in the common bilaterian ancestor and (b) that both vertebrates and cephalopods recruited the evolutionarily conserved pathway and expanded their tubular networks using this genetic pathway.

So we have another example similar to prestin, where extensive anatomical and physiological similarities independently emerged due in part to drawing from the same genetic toolkit.  Convergence is not simply about similar selection pressures, as conventional thinking would have us think, but also derives from using the same, ancient genetic toolkit.  In essence, convergence is a function of preadaptation.

Oh, and if you are wondering, if you probe the sequence data bases, there are homologs of VEGFR in various single-celled protozoa.  Because of all these ancient toolkits and convergence, Stephen Jay Gould’s view of evolution, where the rewound tape of life would supposedly spawn a radically different world, is looking less and less tenable.  On the contrary, it looks as if brains, eyes, ears, hearts, and blood vessels were all bound to emerge give enough time.  It was just a matter of time.

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