W7 by Ariel

8/7/2019 W7 by Ariel

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Week 8

Evolution of Flight Lecture(s):

Big question: Did flight evolve from the top-down (gliders -> powered flight) orbottom-up

(flapping = preadaptation for something else, like navigating rough terrain)?

Why go into the air?

Escape predators Increase land area able to cover

a. Less competition for resourcesb. Increased access to resources e.g. seasonally available food items (esp. for

migrating species like birds and butterflies).

Optimal foragingTypes of flight

(Note: these are roughly order of energetic expense, not necessarily in order of evolution)

1. Passive = moved by the wind; pollen, spiders, microbes (evidence of microbes in theinner continents very early on in lifes history, probs got there by wind)

2. Parachuting fall to the ground with less than a 45 degree angle from launch point;found in fish, amphibians, mammals, and reptiles/birds

3. Gliding fall to ground with more than 45 degree angle from launch point; flyingsquirrels and marsupials, Draco lizards with rib projections

4. Power flight much more expensive than gliding because it requires a power stroke; e.g.bats and lots of birds

5. Gliding extreme type of power flight, not maneuverable and very big; e.g. vulturesRequirements for flight

Have to overcome drag and weight with lift and thrust:

Big lift muscles Limit drag (aerodynamics) Accommodate various speeds

Maintain stability

Morphological features of powered flight

Keeled sternum (muscle attachment site for power stroke, lifting muscles) Modified shoulder girdle for power flight motion Wing has rigid support Large humerus (more surface area of wing = more lift)


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Interesting factoids!

Sexual characteristics and flying go hand and hand in birds and Dracos probably also pterosaurs

because once youre in the air, camouflage is less of a thing.

1/5 of mammals are bats! (The rest is pretty much rodents.) Micro and mega-bats are very

different. Mega-bats dont echolocate, are less maneuverable and rely more on vision becausethey eat fruit. Micro-bats have teeth specialized for eating insects. Bats live for a long timedespite small body size. They also migrate.

Whats up with Ratites and losing flight? Imagine what theyre like with keys

Ratites are big ol flightless birds. Seems like they shouldve lost it just once and then radiated,but tinamous CAN fly and it seems unlikely that they would regain flight again after losing it.

Because of where tinamous fall in the phylogeny (known from genetics), ostriches, rheas, andmoas all had to lose flight independently. The remaining ratites, emus (Australia), cassowaries

(PNG, Australia), and kiwis (New Zealand) are dispersed across non-significant spans of ocean,so if their MRCA lost flight, then they must have rafted by chance to these islands (and theyre

sort of big). Otherwise they must have been able to fly to these islands and again each of themlost flight independently.


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Evolutionary Development LectureWhat molecular biology can tell us about the early origins of animals:

Figuring out animal relationships is the first critical step to understanding animal evolution

Molecular bio has helped us do this started with comparing single ribosomal genesbetween species, now genomics allows us to compare suites of genes

Here are the major phyla were still figuring out their relationships!

B = bilateral, P = protostomes (dual mouth/anus), D = deuterostomes (separate mouth and anus)

Diversification happened sometime (early) in the Protoerozoic most are represented as fossilsin the Cambrian 540mya. Also most Cambrian fossils can be places in existing phyla. Dont have

many pre-Cambrian fossils.

Ecdysozoa (includingarthropods) unified byability to shed skin

Deuterostomes have largestvariation in body forms ofall phyla.

Ambulacraria contains

echinoderms andhemichordates:

Hemichordates like acorn

worms are bilateral andhave 3 parts: proboscis,

collar, and trunk.

Adult echinoderms have lostbilateral symmetry


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So what else can we use besides fossils?

- Molecular clock dates separation of deuterostomes around 900mya!

- Morphology, such as the developmental processes that give rise to adult morphologies =developmental biology. Involves both molecular and cellular processes.

Cell division, movement, specialization i.e. how axis formation occurs along A-P, D-VHox genes pattern the anteroposterior axis in bilateral animals

Hox genes are transcription factors (i.e. affect DNA, activate and inhibit other proteins that are

often themselves transcription factors) to confer fates (e.g. eye, limb, brain) on sections of cellsin a developing embryo. Unique in that their order on the chromosome corresponds to where

they are expressed along the A-P axis. Found in both mammals and fruit flies, which separatedabout 600mya (along with many other things).

The transcription factors and signaling factors involved in patterning structures have

classically been used to compare groups of animals. We can now use the role of these genes ascharacters in a new data set to test hypotheses of homology between structures.

The AP axis is derived from the oral-labial axis in cnidarians. Vertebrates have 4 duplications,

but clusters of genes are lost in some of these copies (see above). Therefore Urbilatera (ancestorof bilaterans) must have had complex molecular cell-patterning systems.


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But we have to be careful about homology of genes and homoplasies in their functionEvolution tends to co-opt genetic circuits rather than make new ones. Therefore convergence

must be considered in these analyses.

Co-option of the hox complex:Posterior most Hox genes make finger and toe buds as well as genital buds (i.e. similardevelopment, but penises came way after limb buds evolutionarily).Posterior few Hox genes pattern A-P axis of internal female reproductive system (i.e. in

addition to body A-P)

There are some crazy similarities between phyla

Anterior brain of flies patterned by same genes as anterior brain of vertebrates; for example, fly

ventral nerve chord patterned by same genes as the vertebrate (dorsal) spinal chord. Despite hugemorphological disparity between vertebrates and fly nervous systems, conserved molecular

processes regulate them.

So what does this information actually tell us about the ancestral characters of stem bilaterians?Such as the complexity of the ancestral nervous system?

BUT the chordate (mouse, human) and arthropod (fruit fly) both have life histories tied to a

centralized nervous system - so possible convergence that would skew our view of commonancestor argument for more model organisms such as his experiments with a hemichordate.

[Summary so far: A/P axis patterning genes are homologous, but we have to worry about

functional convergence when making evolutionary hypotheses about morphology from thedataset of molecular bases of development.]

How can we infer more about the nervous system of the bilaterian ancestor?

Hox genes are used to pattern the CNS of arthropods and chordates use Hemichordate acornworm to get more information about such an ancestor. The acorn worm does not have a

centralized nervous system proboscis is about 60% neurons in a nerve net organization.


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Experiments with acorn worms:As hemichordates develop into adults, can mark which patterning genes to see when and where

they are expressed. Results:1.Hox genes expressed in similar A->P pattern as in chordates and arthropods (repping the

Ecdysozoan phyla).

2.Genes involved in patterning fore, mid, and hind brain similar to chordates and arthropods.3.Hox gene expression for nervous system is in circumferential rings not localized on thedorsal or ventral side as in chordates and arthropods respectively, reflecting organization

of the nerve net nervous system in hemichordates.

Discussion

The same genetic patterning mechanisms that are used to pattern CNS are also used topattern nerve nets. So these genes are conserved homologous good markers of CNSsystems i.e. NOT a result of convergent evolution centralizing the nervous systems in

chordates and arthropods because same genes are at work in distantly related organismwith nerve net CNS organization.

These data (transcription factor/ patterning signals expression like Hox genes) give us aunique way of comparing regions of chordates and hemichordates

We still need more model organisms to say for sure whether the ancestor of bilaterians had

a centralized nervous system or a nerve net:

Need more model organisms like lophotrochozoans, which will give us more info about

protostomes and echinoderms, more info on deuterostomes (see phylogenies below).Echinoderms will be tricky because their Hox gene order is mixed up on the chromosome.


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OR

If the bilateral ancestor had a centralized CNS with a nerve chord when did the

dorsal/ventral axis flip? Is there evidence that it did?

Fruit fly here

Fruit fly here

(On ventral side)

(On dorsal side)


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Inversion Hypothesis: if you compare the dorsoventral axis of arthropods and chordates, themain organ systems are reversed. At some point on the lineage leading to chordates, the

ancestor flipped over on its back reorienting its dorsoventral axis:

Prof. Lowe begrudgingly admits theres molecular evidence for this hypothesis. The samesecreted pathway defines the position of the CNS on the dorsal side in chordates and on the

ventral side in arthropods.Chordin promotes nervous systemBmp antagonizes chordin and promotes epidermis/skin

So what about in Hemichordates? (This will assist in dating this potential flip)

Hemichordates have ventral mouth and dorsal gill slits and strong DV polarity set up. Molecular

experiments show chordin on ventralside and bmp on dorsalside. Expression pattern like flies(protostomes) means if bilaterian ancestor had centralized nervous system, the flip would havehappened after the chordate clade split from the hemichordate-echinoderm clade.

But not so fast bmp doesnt suppress neurons in acorn worms (remember nerve net situation).

Herein lies the question: centralized CNS ancestor and then a flip? Or a nerve net ancestor thenchordin/bmp antagonism convergent evolution setting up DV axis in chordates and flies? In the

words of Julia Childs: WHO KNOWS?! In the words of Prof. Lowe: more model organisms!

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