Chapter 19

Question 1

Hox genes share 3 major traits with all major animal phyla . What are these three traits?

Answer 1

1. They occur in groups (likely caused by gene duplication events)
2. Perfect correlation between 3’-5’ order of the genes along the chromosome and the anterior to posterior products of the genes in the embryo
3. Each gene in the complex contains a highly conserved sequence called a homeobox.

Question 2

What is a homeobox and why is it important?

Answer 2

Highly conserved sequences found in the hox genes and that the products of the hox genes are transcription factors that bind to DNA and control the transcription of other genes

Question 3

The function of homeotic genes is to

Answer 3

define where cells are within the embryo.

Question 4

Hox genes are an example of _______ genes.

Answer 4

homeotic genes

Question 5

Changes in the expression of homeotic genes can lead to

Answer 5

changes in the organization of the body.

Question 6

Once activated, a Hox gene product activates genes responsible for

Answer 6

making the structures appropriate for each location in the embryo.

Question 7

Because Hox genes are so important for organizing the body, and because the number of Hox genes varies among animals, it is logical to hypothesize that changes in Hox gene numbers

Answer 7

triggered evolutionary changes in morphology.

Question 8

Correlations between the morphology of different groups of organisms and the number and types of Hox genes they contain reveals several important points: What are these?

Answer 8

Some genes in the complex appear to be ancestral.

Other genes are probably descended by duplication from these original
genes.

Sponges and Cnidarians have 5 or fewer Hox genes that are not clustered, suggesting that a major change in the number and organization of Hox sequences occurred sometime before the Cambrian explosion.

Question 9

What does Sponges and Cnidarians having 5 or fewer non clustered Hox genes suggest?

Answer 9

This suggests that a major change in the number and organization of Hox sequences occurred sometime before the Cambrian explosion.

Question 10

Current consensus is that changes in the number of Hox genes were important in the diversification of animals, but that in some or many lineages, _________________________________ were perhaps even more significant.

Answer 10

changes in how the available genes were regulated

Question 11

What are the major themes of evolutionary development?

Answer 11

1. Development is controlled by networks of interacting genes.
2. Once a developmental pathway exists for a structure it is elaborated and modified during the course of evolution to produce an array of phenotypes
3. These networks are often shared by many species and are modified to produce variations of the same trait.

Question 12

Natural selection can only act on variation in ______ traits?

For example, the limbs of bats, seals, humans, and horses (and many other animals) all contain variations on an arrangement of bones that existed in the ____________
years ago.

That arrangement of bones was encoded by a cascade of
______ and ________ that were modified over time, by ____________ and _________, to produce the diversity
of limbs observed today.

Answer 12

Existing

fins of lungfish-like organisms 375 million

developmental signals and transcription factors; mutation and natural selection

Question 13

homology

Answer 13

traits inherited by two different organisms from a common ancestor

Question 14

analogy

Answer 14

similarity due to convergent evolution, not common ancestry

Question 15

The underlying similarity we see in

homologous structures is due to

Answer 15

homologies in the genes that are active as those structures develop.

Question 16

Evidence for deep homology in genes

Answer 16

Pax6 genes in mice and fruit flies (Drosophila):
Function early in development in the formation of the
camera eye in vertebrates, and the compound eye of
insects.
When a mouse Pax6 gene is inserted into a fruit fly
embryo, a fly eye forms wherever the mouse gene
happens to be expressed during early development.

Question 17

What is the difference between the Pax6 genes in mice and fruit flies (Drosophila):

Answer 17

Function early in development in the formation of the
camera eye in vertebrates, and the compound eye of
insects.

Question 18

Evolutionary change occurs when genes involved in _____________ are expressed at new times or locations, or in new amounts or durations.
Traits that serve one function can therefore evolve to serve an entirely new function.

Answer 18

regulatory cascades

Question 19

_______ are one of the most striking innovations of the Cambrian explosion, resulting in, for example___________.

Diversification of _______ widely considered key to ecological and evolutionary success of arthropods and terrestrial vertebrates.

Answer 19

Limbs; , fly wings/crab claws; bat wings/
horse legs

limb structure

Question 20

Three basic elements of The Tetrapod Limb

Answer 20

1. Stylopod (called the humerus in the forelimb and femur in the hind limb)
2. Zeugopod with two bones (radius and ulna or tibia and fibula)
3. Autopod consisting of small elements (carpus or wrist, tarsus or ankle) and the digits (fingers or toes)

Question 21

Signiture adaptation

Answer 21

Tetrapod Limb

Question 22

What are the Major types of genetic control over limb formation in arthropods

Answer 22

1. The decision whether to make a limb is determined by a gene called wingless.
2. The decision to extend the limb distally hinges on the
   expression of a gene called Distal-less (Dll).
3. The decision on which type of limb will develop is controlled by homeotic genes.

Question 23

Changes in the regulation of the gene ____ in particular might lead to new or modified limb outgrowths.
Experiments show that variation in the timing or location of __________ appears to correlate with variation in the number, location, and shape of arthropod limbs.

Answer 23

Dll

Dll expression

Question 24

Q: How is a zygote transformed into an adult organism?

Answer 24

A: Embryos start out as a mass of cells.
As development continues, cells in each part of the embryo
begin expressing distinct subsets of the genome and
differentiate into recognizable and functioning muscle,
vascular tissue, bone, epidermis, cartilage, or parenchyma.

Question 25

Adult phenotypes vary because…

Answer 25

of variations in development; development varies if a particular signal or transcription factor is produced in a new location, in a different amount, or at a different time.

Question 26

Morphology changes over time because

Answer 26

mutation, selection, and drift act on genes that control embryonic development.

Question 27

The premise of evo-devo research is

Answer 27

that great changes in evolutionary history were caused by mutations that changed developmental sequences and pathways.

Question 28

Early evo-devo research on Drosophila showed

Answer 28

That specific genes are involved in specification of the anterior-posterior body axis and the organization of the segmented body plan.

Question 29

What did scientists begin to notice that eventually lead to Evo-Devo?

Answer 29

Species from widely divergent branches on the animal phylogeny, fruit flies and house mice, appeared to build their segmented bodies in similar ways

Question 30

The development of ______ made it

possible to look for and study homologues of these genes* (aka ________ ______) in an array of different species.

Answer 30

PCR; candidate genes

Question 31

_______ ____ are active in gene networks that control the overall organization of an animal or plant body, including its size and shape.

Answer 31

Homeotic genes

Question 32

2 important homeotic loci:

Answer 32

Bithorax complex (BX-C)
Antennapedia complex (ANT-C)

Question 33

A mutant bithorax gene yields flies with…

A mutant Antennapedia gene yields flies with….

Answer 33

4 wings instead of the normal 2.

legs in place of antennae.

Question 34

What is interesting about a fly with 4 wings?

Answer 34

The 4-winged fly is interesting because fossil and molecular phylogenetic data suggest that flies evolved from ancestors with 4 wings; this observation is
consistent with the hypothesis that changes in Hox gene expression were responsible for this change.

Question 35

Correspondence between genomic order of Hox gene loci and their spatial locations of expression along the body axis is known as …

Answer 35

spatial colinearity.

Question 36

Genes in different species derived from a common ancestor’s gene are said to be…

Answer 36

orthologous.

Question 37

Genes within a species that arise via duplication are said to be

Answer 37

paralogous.

Question 38

Traits shared because they were present in, and inherited from, a common ancestor are called…

Answer 38

homologous.

Question 39

Similar features in two organisms that were not present in, and inherited from, their most recent common ancestor is…

Answer 39

homoplasy.

Question 40

Development is the process by which

Answer 40

the genotype, in coordination with the environment, produces an organism’s phenotype through the life cycle.

Question 41

Parallel evolution -

Answer 41

where similarity results from the same underlying developmental mechanism.

Question 42

Convergence -

Answer 42

similarity resulting from a different underlying developmental mechanism.

Question 43

The same genes seem to function repeatedly at different times and places during development is known as

Answer 43

pleiotropy.

Question 44

trade-off

Answer 44

where one feature of an organism

can be promoted only at the expense of another.

Question 45

An example of when evolutionary change proceeds in
discontinuous bursts (example of evolution by leaps)  is...

Answer 45

cross-species hybridization in plants, where pollen from one species fertilizes the ovum of a another, yielding a potential third species.

Question 46

A second mechanism for evolutionary leaps is horizontal gene transfer, in which

Answer 46

foreign DNA integrates stably into a new genome.

Question 47

What are the implications for Hox gene evolution?

Answer 47

If segmentation in arthropods and chordates is an example of homoplasy, then the similar function of Hox genes in regulating segmental identity in vertebrates and insects is an example of parallel evolution, perhaps exapted
from a regionalized expression of Hox genes in the central nervous system of some worm-like bilaterian ancestor.