F Rosa Rubicondior: This Week's Accidental Evidence for Evolution

Saturday 22 September 2018

This Week's Accidental Evidence for Evolution

Octopus bimaculoides
Credit: Tom Kleindinst/Marine Biological Laboratory
Octopuses Given Mood Drug ‘Ecstasy’ Reveal Genetic Link to Evolution of Social Behaviors in Humans.

As I've pointed out many times, biologists these days don't set out to find evidence for evolution any more so than physicists set out to find evidence for the Laws of Thermodynamics nor do chemists set out to prove the Atomic Theory. It just happens, because the theories are true, that whatever they do confirms the underlying theories.

This week for example we have a couple of papers that yet again casually confirm that the TOE is true and the only way to make sense of what can be seen.

The first is a paper which seems to show that social behaviour has a genetic basis that goes back at least half a billion years to the common ancestor of octopuses and humans. Although these finding are only preliminary it shows how the same genes are present in widely separated species - which also suggests they are sufficiently advantageous as to be highly conserved during hundreds of millions of years of evolution.

The brains of octopuses are more similar to those of snails than humans, but our studies add to evidence that they can exhibit some of the same behaviors that we can. What our studies suggest is that certain brain chemicals, or neurotransmitters, that send signals between neurons required for these social behaviors are evolutionarily conserved.

Gül Dölen, M.D., Ph.D. Lead investigator.
Assistant professor of neuroscience at the Johns Hopkins University School of Medicine.
Gül Dölen, assistant professor of neuroscience at the Johns Hopkins University School of Medicine and Eric Edsinger, a research fellow at the Marine Biological Laboratory in Woods Hole, Massachusetts, found that octopuses and humans had nearly identical genomic codes for the transporter that binds the neurotransmitter serotonin to the neuron’s membrane.

Serotonin is a well-known regulator of mood and closely linked to certain kinds of depression. This transporter is known to be the site where the drug MDMA or "ecstacy" binds to brain cells and caused mood changes, inducing so-called pro-social behaviour in humans, mice and other vertebrates.

Octopuses are known to be solitary rather than social animals and are also known to be highly intelligent.

The Johns Hopkins Medical Centre press release explains:

Dölen designed an experiment with three connected water chambers: one empty, one with a plastic action figure under a cage and one with a female or male laboratory-bred octopus under a cage.

Four male and female octopuses were exposed to MDMA by putting them into a beaker containing a liquefied version of the drug, which is absorbed by the octopuses through their gills. Then, they were placed in the experimental chambers for 30 minutes. All four tended to spend more time in the chamber where a male octopus was caged than the other two chambers.

“It’s not just quantitatively more time, but qualitative. They tended to hug the cage and put their mouth parts on the cage,” says Dölen. “This is very similar to how humans react to MDMA; they touch each other frequently.”

Under normal conditions, without MDMA, five male and female octopuses avoided only male, caged octopuses.

Dölen suggests that the neurocircuitry for social behaviour is present but is suppressed under normal situations, being activated for mating, but suppressed again soon after when the behaviour reverts to aggressive, asocial mode.

  • Phylogenetic analysis revealed clear octopus orthologs of human SLC6A4
  • SLC6A4 protein alignment revealed conservation of the MDMA binding site in octopuses
  • A novel assay was developed for quantification of octopus social behaviors
  • Behavioral analysis revealed conservation of prosocial function of MDMA in octopuses

Human and octopus lineages are separated by over 500 million years of evolution [1, 2] and show divergent anatomical patterns of brain organization [3, 4]. Despite these differences, growing evidence suggests that ancient neurotransmitter systems are shared across vertebrate and invertebrate species and in many cases enable overlapping functions [5]. Sociality is widespread across the animal kingdom, with numerous examples in both invertebrate (e.g., bees, ants, termites, and shrimps) and vertebrate (e.g., fishes, birds, rodents, and primates) lineages [6]. Serotonin is an evolutionarily ancient molecule [7] that has been implicated in regulating both invertebrate [8] and vertebrate [9] social behaviors, raising the possibility that this neurotransmitter’s prosocial functions may be conserved across evolution. Members of the order Octopoda are predominantly asocial and solitary [10]. Although at this time it is unknown whether serotonergic signaling systems are functionally conserved in octopuses, ethological studies indicate that agonistic behaviors are suspended during mating [11, 12, 13], suggesting that neural mechanisms subserving social behaviors exist in octopuses but are suppressed outside the reproductive period. Here we provide evidence that, as in humans, the phenethylamine (+/−)-3,4-methylendioxymethamphetamine (MDMA) enhances acute prosocial behaviors in Octopus bimaculoides. This finding is paralleled by the evolutionary conservation of the serotonin transporter (SERT, encoded by the Slc6A4 gene) binding site of MDMA in the O. bimaculoides genome. Taken together, these data provide evidence that the neural mechanisms subserving social behaviors exist in O. bimaculoides and indicate that the role of serotonergic neurotransmission in regulating social behaviors is evolutionarily conserved.

The second paper deals with the local evolution of the ubiquitous house sparrow.

House sparrow, Passer domesticus.
Sparrow in a Lead Mine: Birds Adapt to Life in Contaminated Areas | Macquarie University.

The house sparrow, Passer domesticus, is a commensal species that lives alongside humans and is entirely dependent on us, being almost unknown in the wild. It has proved to be readily adaptable and has spread with humans across all inhabited continents, climates and latitudes, so making it an ideal subject for studying its adaptation to a wide range of environments.

This paper shows that the sparrows around the Australian lead-mining areas of Broken Hill, New South Wales and Mount Isa, Queensland, introduced 100 and 50 years ago respectively, have evolved to live in the presence of high levels of lead in their environment. Sadly, Elsevier want money for permission to reprint the abstract. Because of their remoteness, these populations are effectively genetically isolated.

A study led by researchers from Macquarie University found 12 genetic variations in the sparrows from the mining areas that have previously been associated with lead exposure in laboratory conditions. These genes are associated with two metal transporters that are involved in the transport of metals, including lead and zinc across cell membranes.

The suggestion is that the presence of these variant genes brings about a down-regulating of the lead transporters on the surface of the body helping to protect the sparrows from environmental contamination and reducing the harmful effects on neural and bone development and preventing lead from accumulating in the body.

Here then we have two examples from the many biology papers published this week in which evolution is the explanatory theory that makes sense of what we see. Octopuses and humans sharing ancient, highly-conserved genes that regulate social behaviour only makes sense as the product of divergence for a common ancestor half a billion years ago. And genetic diversity in the house sparrow in response to environmental contamination with toxic heavy metals is a predictable outcome of the Theory of evolution. Mutations which enable the sparrows to live in contaminated areas will be expected to increase in the isolated gene pool and this is precisely what we see at Broken Hill and Mount Isa.


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