Limblessness and Venom Evolution

Question 1

Largest group of repltiles?

Answer 1

squamates (snakes and lizards)

Question 2

Limblessness as a continuum in squamates?

Answer 2

• highly reduced limbs in lizards and even examples of remnants of ancestral limbs which are maintained for other functions in some snakes

Question 3

Partial limb loss

Answer 3

limbs usually lost in pairs (e.g. hindlimbs and/or forelimbs, never one of each) - when either forelimbs or hindlimbs are lost the remaining pair are highly reduced

Question 4

Complete Limb Loss - two animals vs eachother

Answer 4

Lizards vs snakes

Question 5

Why limblessness?

Answer 5

limblessness evolved as adaptation to one of two different habitat types: fossorial/burrowing and ‘grass-swimming’ in thick grassland - both habitats = difficult for limbs, interfering with movement by breaking up the smooth body contour that helps push through soil or through grass

Question 6

Moving when legless

Answer 6

4 ways (by limbless squamates):
- Serpentine locomotion - lizards + snakes - ‘S-shaped’ movements whereby body is alternately pushing right to left (directions that cancel each other out) and backwards (pushes the animal forwards smoothly) - most common

• Rectilinear motion - ‘rib-walking’ or ‘caterpillar walking’ - used by heavy bodied snakes + by all snakes to traverse difficult (e.g. smooth and slippery) terrain where friction is limited - ribs connected to the ‘belly’ (ventral) scales of snakes + are moved back and forward in a walking movement to pull the snake along in a straight line
• Concertina movement - involved anchoring one half of the body at a time, i.e. anchoring back half of body then stretching out + anchoring the front half, and pulling back half forward, and repeating this - snakes use this in difficult activities that require grip while moving, e.g. climbing tree trunks or moving through confined space such as prey burrows (where the body is anchored against the sides)
• Sidewinding - to move on shifting sand dunes (though some species also use it on mud flats) where the surface is very low friction and moving itself - involve lifting part of body up (so pushing down against substrate rather than at an angle which would cause slipping), throwing it forward and then putting it down in a new location

Question 7

Coincidental morphological changes along side limblessness

Answer 7

limblessness evolves with other traits - forming snake-like body form

1st - body elongation occurs via addition of vertebrae (controlled by Hox genes)
2nd - limbs get shorter and generally smaller - as limbs decrease in size, number of digits also reduces, usually lost in specific pattern (some variation has been documented)

Question 8

What are ecomorphs?

Answer 8

morphological differences that are related to the ecology

Question 9

Example of ecomorphs in snakes and lizards

Answer 9

2 evolutionary drivers to limblessness - burrowing vs grass-swimming - result in two different body plans (Brandley et al., 2008)

These morphological differences are related to ecology and so are called ‘ecomorphs’

Question 10

What does the dichotomy of burrowing vs grass-swimming ecomorphs suggest?

Answer 10

ancestral snake likely was a burrower and that this lifestyle was responsible for the evolution of limblessness in that group

Question 11

synapomorphies

Answer 11

shared derived traits

Question 12

Problem for squamate phylogeny

Answer 12

reconstructing phylogenetic trees seek derived states that are shared by descendants - provide evidence that taxa sharing them have more recent common ancestor than other taxa - more synapomorphies a set of taxa share, the stronger the support for them being part of single clade

H/E lots of traits may evolve together - e.g. evolution of limblessness - can mislead phylogenetic inference by suggesting lots of support from each individual trait while ignoring fact that if one of those traits changes so will all the rest because they are linked

Question 13

What is debated?

Answer 13

evolutionary history of squamates
- traditional phylogeny (based on morphology) differs in important ways from newer phylogenies based on molecular data

major change = Iguanians either at very base of squamate tree (morphology) or highly derived and forming a group called ‘Toxicofera’ alongside snakes and another group of lizards called ‘anguimorphs’ (based on molecular data)

not only influences our understanding of the phylogenetic relationships, but also understanding of evolution of traits of these animals , and so has widespread consequences for their biology

Question 14

Debate Resolved

Answer 14

(Wiens et al., 2006)
- combining both molecular and morphological traits together into one analysis - looking for patterns in which traits evolve together, and ‘weighting’ the support to give the phylogeny accordingly

• traits associated w/ limblessness found to evolve together and when this convergent evolution of a whole suite of traits was accounted for, the ‘molecular’ phylogenetic tree was recovered even with the rest of the morphological data

Question 15

What is venom?

Answer 15

definitions vary

broadly speaking - toxic substance which is produced by one animal and transferred to another via a wound

highly complex mixtures of individual molecules (toxins) which have a range of effects on the envenomated animal

Question 16

What does venom do?

Answer 16

3 major types of functional activities of venoms

1. Cytotoxicity - damages cells and consequently tissues - Atractaspis bibronii
2. Haemotoxicity - interferes with the circulatory system including blood coagulation - promotes bleeding - Daboia russelii
3. Neurotoxicity - interferes w/ nerve transmission + causes paralysis or other neurological symptoms - often most serious effects from a clinical perspective - Naja naja

Question 17

What is Venom used for?

Answer 17

• predation (incapacitating prey)
• defence against predators
• competition w/ conspecifics
• reproduction during courtship

predation & defence = most common

competition = often being a function of mammals venom

courtship used restricted to few arthropods such as flat rock scorpions

primary selective pressure on a venom might be for one function but the venom can also function and be used in another - e.g. snake venoms are selected for prey capture but effects on humans suggest their subsidiary use in defence

Question 18

Where do toxins come from?

Answer 18

• evolved from existing proteins in body
• e.g. ADAM metalloproteases = group of enzymes that are embedded in cell membranes - ‘cut’ membrane proteins in specific ways that either release proteins from cell or activate membrane receptors - these enzymes are therefore common throughout animals and vital to normal function of cells - H/E mutations which change the expression and function created a group of toxins called ‘snake venom metalloproteases’ which also ‘cut’ proteins, but do so less specifically and as result break down tissue in envenomated animals

this pattern of co-opting existing body proteins and tweaking their function to create effective toxins is a general pattern in venom evolution

Question 19

Convergent Evolution of Venom

Answer 19

• extensive convergence across animal phylogeny
• of 4 classes of tetrapods - birds have not evolved venom at all
• convergence = evidence in toxin types - not just overall presence of venom itself
• venom evolved at least once in most major animal groups
• convergent evolution = common on all levels of venom evolution, from particular toxins used to the presence of venom in general

Question 20

Reptile Venom: the Toxicofera

Answer 20

Toxicofera clade (from conflict over sqaumate phylogeny) was named because it contains all known venomous reptiles

investigations of oral glands across many species in this clade, suggested that distribution is best explained by single origin of reptile venom systems (at the base of the Toxicofera clade), implying that non-venomous toxicoferans have lost venom

traditional assumption = venom evolved several times in snakes and at least once in lizards - shift in interpretation of reptile venom evolution

Major shift in interpretation of the evolution of venom in squamate reptiles
suggests venom = more widespread in this group than previously thought
suggests that members of this group that are not venomous must have lost the ability to produce venom again

despite evidence from wide range of types of data, this idea has been debated, particularly in case of species where venom does not pose a medical problem to humans, such as Komodo dragons

resolution of debate has not yet happened, reflecting dynamic nature of science + the cautious scepticism towards new ideas