Lecture 29: Development and Evolutionary Change II

Turtle shell experiment (heterotopy)

The carapacial ridge (cr) secretes a growth factor, Fgf10, to which the rib responds by secreting Fgf8, a case of mutual induction. This way, the ribs grow toward the cr and so fail to grow ventrally as it does in other tetrapods (G. gallus in the sketch, the chicken). This is how the turtle "shell" forms, which is scaffolded by the "misgrown" ribs. In turtle, the ribs stop at the carapacial ridge --> don't grow onto the ventral side

Uses of eukaryote flagella/cilia (developmental constraints- physical)

Developmental constraints a) Physical

Eukaryotic Flagella (developmental constraints - physical)

Developmental constraints a) Physical Eukaryotic cilia and flagella have very few components, consisting of an array of microtubules paired up in 9 doublets surrounding a central doublet (called axoneme here, which I think is wrong). This arrangement is highly conserved and thus universal. Bending of the cilium or flagellum is caused by the doublets rubbing past each other along their lengths. This motion is mediated by dynein arms that engage the tubulin monomers in the microtubule in the presence of ATP. Basically, there seems no better way to arrange microtubules and have them work so elegantly and well. What are the uses of these organelles? --> All cilia/flagella of eukaryotes look exactly the same (invariant structure)

Drosophila sperm (developmental constraints - physical)

Developmental constraints a) Physical: the remarkably long sperm of Drosophila It is a truism among reproductive biologists that the larger the sperm, the more competitive they are. The winner in this "lottery" is Drosophila bifurca. The adult male is shown on the left, surrounded by his own testes, one of which is stretched out to show off its length. That length accommodates sperm, shown in close-up on the right: each jumble of threads is one sperm, so you're looking at two sperm inside a seminal vesicle. Apparently, because there doesn't seem to be other ways of building a flagellum, the only way to make a giant sperm is to increase the length of the flagellum!

Tooth development Part 2 (developmental constraints - morphogenetic)

Developmental constraints a) Physical: the remarkably long sperm of Drosophila b) Morphogenetic: ------> i) Signaling centers during tooth development These are morphogenetic constraints b/c you really don't have room for that many signaling centers, and the signaling centers predict where the teeth are going to grow! There seems to be just a limited number of ways to build a molar, and these depend on the signaling centers that form during tooth development. Molar teeth (shown here in an animal with three molars) develop front to back (M3 --> M1). Tooth size is predicted by the ratio (p) between an activator (a, Sox 2) and an inhibitor (i, Shh). If p is low, one gets a large posterior molar (M1), and if p is high, one gets a large anterior molar (M3). That's interesting enough. However more interesting is the fact that the size of M2 will always be 1/3 the combined sizes of the three molars. Thus, the growth of teeth is dictated by morphogenetic constraints.

Tooth development Part 1 (developmental constraints - morphogenetic)

Developmental constraints a) Physical: the remarkably long sperm of Drosophila b) Morphogenetic: ------> i) Signaling centers during tooth development Tooth development is shown here as an inductive event; the interaction between the ectoderm (pink) and the underlying dermis (blue, with blood vessels) produces signaling centers that dictate the shape of the final tooth that forms. These centers (blackened tips) express Sox2 and Shh, and predict the possible tooth shapes one can expect.

EXAM 5 BONUS QUESTION! (developmental constraints - morphogenetic)

Developmental constraints a) Physical: the remarkably long sperm of Drosophila b) Morphogenetic: ------> i) Signaling centers during tooth development ------> ii) Why do tetrapods have five fingers as the norm (Bonus exam question) What is the developmental constraint that prevents us from having more or fewer than 5 fingers? It sort of seems like based on the articles that we don't know the answer? MY NOTES: https://docs.google.com/document/d/1Y2cv6kHxinUsGMHeF9yDuoUctvxYNcABtNWilDMc9jw/edit https://www.scientificamerican.com/article/why-do-most-species-have/ https://www.nationalgeographic.com/science/phenomena/2014/08/01/how-did-you-get-five-fingers/

Developmental Constraints - Phyletic Part 1

Developmental constraints a) Physical: the remarkably long sperm of Drosophila b) Morphogenetic: ------> i) Signaling centers during tooth development ------> ii) Why do tetrapods have five fingers as the norm (Bonus exam question) c) Phyletic: Is there a phylotypic stage? Early Embryogenesis: Embryos start off very simply (not many genes being expressed). Mid Embryogenesis: At a slightly later stage, we have built many more GRNs --> a natural consequence of the embryonic genome kicking in --> considered the "phylotypic stage" Late Embryogenesis: 'Explosion' in terms of divergence (released from phyletic constraint) The notion of a phyletic constraint refers to the restrictions that result from an organism's developmental history. During early embryogenesis, the molecular networks are few and simple, arriving at a so-called phylotypic stage (developmental stage with a look-alike appearance that is common to all the members of that phylogenetic group. This is the stage that corresponds to "mid embryogenesis."

Precocious development (heterochrony)

Heterochrony refers to evolutionary changes in the timing of some developmental event. a) Precocious vs. altricial development Changes the timing of the developmental event of gestation/birth Compare the advantages and disadvantages of altricial development with precocious development as typified by the newborn giraffe, shown here getting ready to stand up for the first time. Precocial = hatched or born in an advanced state and able to feed itself almost immediately (basically "all there" structurally upon being born) - Precocious development confers evolutionary advantages b/c the young are all ready to go, except gestation is longer. - With altricial development, animals can have more children in one breeding period which is advantageous in case one of the young dies

Altricial development (heterochrony)

Heterochrony refers to evolutionary changes in the timing of some developmental event. a) Precocious vs. altricial development The neonate of the red kangaroo is perfectly capable of crawling, although its hindlimbs are significantly immature. Its olfactory system is also in place, enabling it to navigate the distance between the cloacal opening and the maternal nipple field. All other organ systems are immature; this is altricial development. Altricial = hatched or born in an undeveloped state and requiring care and feeding by the parents.

Bird vs. Human Skulls (heterometry)

Heterometry refers to evolutionary changes in the rate of development, resulting in different proportions. a) Skull bones grow at different rates (allometry) Note that the component bones of the avian and hominid crania are highly similar especially in their positions (with respect to each other). What is different between them is the comparative rates at which the individual bones grow, which result in their allometric (non-similar) proportions. This is an example of heterometry (different rates of growth) which can bring about significant evolutionary divergence.

Crabs, Termites, and Ants (heterometry)

Heterometry refers to evolutionary changes in the rate of development, resulting in different proportions. a) Skull bones grow at different rates (allometry) b) Body regions grow at different rates (polymorphism within a species) Allometry describes the different patterns of growth of different body parts in individuals of the same species. In fiddler crabs, the male commonly has a larger left arm. Among termites and ants, the different castes shown exhibit anatomical features that result from different patterns of growth of the different body parts (appendages, body regions). Allometry explains polymorphism. In the queen termite, the soft tissue between the strips swells so she becomes a huge, egg-laying machine. The soldier has a hardened head capsule.

Turtle shell explanation (heterotopy)

Heterotopy refers to evolutionary changes in the location of some developmental event. a) How the turtle got its shell Note the rib in the image of a cross-sectioned turtle embryo. The tip of this rib points toward a lateral protruding ridge of the body called the carapacial ridge (cr). The cr secretes a growth factor, Fgf10, to which the rib responds by secreting Fgf8, a case of mutual induction.

Why snakes don't have legs (heterotopy)

Heterotopy refers to evolutionary changes in the location of some developmental event. a) How the turtle got its shell b) How the snake lost its legs The absence of limbs in modern snakes is attributed to the expression pattern of hox genes. Note how in the snake, most of the vertebrae express both Hoxc6 and Hoxc8, making them nearly all thoracic. Compare this expression pattern with that in a tetrapod. How does this explain the origin of snakes from a legged ancestor? How does this explain the following unusual 1-legged snake?? Snakes make "too many" thoracic vertebrae, and thoracic vertebrae don't bear limbs --> their unique patterning by Hox genes does not support limb formation There was a change in the location of the developmental expression of hox genes. Hoxc6 and Hoxc8 ended up being expressed in most of the snake body, leading to a lack of legs

Insects and Abdominal Legs Part 2 (heterotypy)

Heterotypy refers to changes in developmental outcome arising from mutations in key regulatory molecules that control embryonic pattern formation and tissue specification a) Why adult insects have no abdominal legs Shown here is the amino-acid sequence of the hox protein encoded by AbdA (Abdominal A) in different organisms. Note that the insects have long polyA (polyalanine) inserts in their versions of AbdA. This insert has an interesting function: it represses the gene called Distal-less, which is the "make-a-leg-here" gene in the animals listed here. In insects, Distal-less is thus repressed in the abdomen; and as a result, no adult insects have abdominal legs. By contrast, relatives of insects (the other animals listed in the chart) lack this poly-A insert, and so the leg-making abilities of their respective abdomens remain intact; thus they have abdominal legs. NOTE: Springtails (Collembola) were considered insects once upon a time, too, but do not have the poly-A insert in their AbdA polypeptide. Not surprisingly, they are no longer classified as insects

Insects and Abdominal Legs Part 1 (heterotypy)

Heterotypy refers to changes in developmental outcome arising from mutations in key regulatory molecules that control embryonic pattern formation and tissue specification a) Why adult insects have no abdominal legs key regulatory molecules = e.g. master genes, homeotic genes, BMP, etc. AbdA polypeptide with polyA insert represses distal-less in the abdomen --> no legs!

Fish Growth (heterotypy)

Heterotypy refers to changes in developmental outcome arising from mutations in key regulatory molecules that control embryonic pattern formation and tissue specification a) Why adult insects have no abdominal legs b) Why fishes keep growing in size as they age Do not memorize, duh. The growth factor called GDF5 (growth/differentiation factor 5) is responsible for the lifelong ability of bony fishes to keep increasing in body size. The amino-acid sequence of GDF5 in many bony fishes is shown here, alongside the tetrapod version. Bony fishes are described as having indeterminate growth; they keep growing as they age. This chart shows how bony fishes have a variably long or extended N terminal region in their GDF 5 amino-acid sequence, whereas tetrapods don't. The capacity for indeterminate growth is linked to the presence of this extended N terminal region. The upper figure is a sketch of the parts of the protein product of GDF5.

Evidence that snakes used to have legs (heterotopy)

If we alter the expression domain of the hox genes to pattern for something other than a thoracic vertebra, we can express a leg in the snake.

Developmental Constraints - Phyletic Part 2

When multiple and independent molecular networks form in each of the organ modules (otic placode, optic placode, tail, heart, limb bud), the multiplicity of these novel networks relieves (erases) the restrictions that forced mid-stage embryos within one phylogenetic group to be so similar in anatomy. And so the embryos within a phylogenetic group start to gain their distinctive anatomies at this point. Organ modules are "independent" - e.g. limb buds will develop in their own way in different species (freed from the phyletic stage of before --> no longer forced to resemble other species)