Chapter 19 Flashcards
Hox genes share 3 major traits with all major animal phyla . What are these three traits?
- They occur in groups (likely caused by gene duplication events)
- Perfect correlation between 3’-5’ order of the genes along the chromosome and the anterior to posterior products of the genes in the embryo
- Each gene in the complex contains a highly conserved sequence called a homeobox.
What is a homeobox and why is it important?
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
The function of homeotic genes is to
define where cells are within the embryo.
Hox genes are an example of _______ genes.
homeotic genes
Changes in the expression of homeotic genes can lead to
changes in the organization of the body.
Once activated, a Hox gene product activates genes responsible for
making the structures appropriate for each location in the embryo.
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
triggered evolutionary changes in morphology.
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?
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.
What does Sponges and Cnidarians having 5 or fewer non clustered Hox genes suggest?
This suggests that a major change in the number and organization of Hox sequences occurred sometime before the Cambrian explosion.
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.
changes in how the available genes were regulated
What are the major themes of evolutionary development?
- Development is controlled by networks of interacting genes.
- Once a developmental pathway exists for a structure it is elaborated and modified during the course of evolution to produce an array of phenotypes
- These networks are often shared by many species and are modified to produce variations of the same trait.
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.
Existing
fins of lungfish-like organisms 375 million
developmental signals and transcription factors; mutation and natural selection
homology
traits inherited by two different organisms from a common ancestor
analogy
similarity due to convergent evolution, not common ancestry
The underlying similarity we see in
homologous structures is due to
homologies in the genes that are active as those structures develop.
Evidence for deep homology in genes
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.
What is the difference between the 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.
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.
regulatory cascades
_______ 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.
Limbs; , fly wings/crab claws; bat wings/
horse legs
limb structure
Three basic elements of The Tetrapod Limb
- Stylopod (called the humerus in the forelimb and femur in the hind limb)
- Zeugopod with two bones (radius and ulna or tibia and fibula)
- Autopod consisting of small elements (carpus or wrist, tarsus or ankle) and the digits (fingers or toes)
Signiture adaptation
Tetrapod Limb
What are the Major types of genetic control over limb formation in arthropods
- The decision whether to make a limb is determined by a gene called wingless.
- The decision to extend the limb distally hinges on the
expression of a gene called Distal-less (Dll). - The decision on which type of limb will develop is controlled by homeotic genes.
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.
Dll
Dll expression
Q: How is a zygote transformed into an adult organism?
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.
Adult phenotypes vary because…
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.
Morphology changes over time because
mutation, selection, and drift act on genes that control embryonic development.
The premise of evo-devo research is
that great changes in evolutionary history were caused by mutations that changed developmental sequences and pathways.
Early evo-devo research on Drosophila showed
That specific genes are involved in specification of the anterior-posterior body axis and the organization of the segmented body plan.
What did scientists begin to notice that eventually lead to Evo-Devo?
Species from widely divergent branches on the animal phylogeny, fruit flies and house mice, appeared to build their segmented bodies in similar ways
The development of ______ made it
possible to look for and study homologues of these genes* (aka ________ ______) in an array of different species.
PCR; candidate genes
_______ ____ are active in gene networks that control the overall organization of an animal or plant body, including its size and shape.
Homeotic genes
2 important homeotic loci:
Bithorax complex (BX-C) Antennapedia complex (ANT-C)
A mutant bithorax gene yields flies with…
A mutant Antennapedia gene yields flies with….
4 wings instead of the normal 2.
legs in place of antennae.
What is interesting about a fly with 4 wings?
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.
Correspondence between genomic order of Hox gene loci and their spatial locations of expression along the body axis is known as …
spatial colinearity.
Genes in different species derived from a common ancestor’s gene are said to be…
orthologous.
Genes within a species that arise via duplication are said to be
paralogous.
Traits shared because they were present in, and inherited from, a common ancestor are called…
homologous.
Similar features in two organisms that were not present in, and inherited from, their most recent common ancestor is…
homoplasy.
Development is the process by which
the genotype, in coordination with the environment, produces an organism’s phenotype through the life cycle.
Parallel evolution -
where similarity results from the same underlying developmental mechanism.
Convergence -
similarity resulting from a different underlying developmental mechanism.
The same genes seem to function repeatedly at different times and places during development is known as
pleiotropy.
trade-off
where one feature of an organism
can be promoted only at the expense of another.
An example of when evolutionary change proceeds in discontinuous bursts (example of evolution by leaps) is...
cross-species hybridization in plants, where pollen from one species fertilizes the ovum of a another, yielding a potential third species.
A second mechanism for evolutionary leaps is horizontal gene transfer, in which
foreign DNA integrates stably into a new genome.
What are the implications for Hox gene evolution?
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.
