Recall HOX genes
Such genes have now been found in all metazoans and some plants and fungi.
Hox gene products regulate the expression of other genes, generally by binding to a 4 bp regulatory sequence adjacent to the other gene.
They provide information about position and time that cues the activation of many other genes.
Hox genes comprise a multigene family, built up by gene duplication.
In addition, vertebrates have 4 sets of Hox genes, probably resulting
from two rounds of duplication of the entire family.
Role in development
Different Hox genes are expressed at different times in devo and in
different regions.
Expressed in order of position:
Those at the 3' end of the chromosome are expressed earlier and more
anteriorly than those towards the 5' end.
For example, here is the pattern of Hox gene expression in an insect.
Note that this is an example of a gene family that appears to have been built up by gene duplication. In this case, simply changing position induces a change in expression.
Expression patterns overlap, providing a "map" of the embryo that other genes can respond to.
Hox gene products are examples of Transcription Factors.
Transcription factors are proteins that bind to DNA and regulate the transcription of other genes.
The short sequence to which a transcription factor binds, which is usually adjacent to the gene being regulated, is called a Cis-Regulatory Element, or CRE for short.
Evolution of gene expression often involves changes in CREs.
For example, in different species of Drosophila, about half of the genetic changes that are responsible for the differences between species are actually changes in CREs.
By contrast, genes encoding for transcription factors rarely change, since any change will simultaneously affect the expression of many different genes.
Most developmental evolution thus results from changes in CREs, changes in the sequences of the genes that CREs regulate, and changes in the parts of the body in which a particular transcription factor is expressed (but not changes in the sequences of the transcription factors themselves).
Example:
In all Arthropods: genes Antp, Ubx, and abdA are expressed in that
order with overlap.
In Crustaceans, there is substantial overlap, with all three genes expressed over most of the body, which is composed of many similar segments.
In Insects, the overlap is reduced (Antp expressed only in the thorax and part of the head, abdA only in the abdomen).
This means that more regions of the insect are distinguishable by the
combination of Hox genes expressed than for Crustaceans.
Correspondingly, Insects have more variable body segments and much
of the morphological diversity in Insects corresponds to local elaboration
of different segments.
There is sometimes more involved than simply change in the range of Hox gene expression.
In Insects, there has been change in the Ubx (Ultrabithorax) gene such
that the protein that it produces acts as a repressor of some other pattern
determining genes.
A consequence of this is that Insect Ubx inhibits limb development
on the abdomen, limiting Insects to three sets of legs.
Ubx protein from crustaceans does not inhibit limb development in insects.
Example: For the Bithorax gene (Ubx), Insects that have 4 wings (most,
other than Dipterans) do not have the same mutation of that gene that produces
4 wings in Drosophila (a Dipteran). They have the same sequence
that produces 2 wings in Dipterans.
Thus, the change from 4 wings to 2 in Diptera did not involve any change
in the Bithorax gene. Rather, it involved many changes in other genes that
Ubx simply influences the expression of.
In the hindwings of Lepidopterans (butterflies and moths), Ubx influences
genes responsible for the shape and pattern of the wings.
{Note, Bithorax flies can not fly. The wings are there but the musculature is insufficiently developed.}
Example: In Drosophila, the ey gene serves
to trigger eye formation. Expressing ey in any part of the flies
body causes an eye to develop there (the sci-fi potential is awesome).
The Pax-6 gene in mice produces a protein that is 94% identical
to the fly's ey protein. Sure enough, expressing Pax-6
in some part of a fly causes a fly eye to develop there.
Pax-6 is associated with eye development in mice, but transplanting
it to another part of the mouse does not lead to extra eyes.
A homologue of Pax-6 is expressed in Cephalopods (octopus, squid, nautilus) eye development.
The common ancestor of mice, flies, and cephalopods did not have an image forming eye, though it probably had light sensitive organs (as nearly all animals do).
It thus appears that the ey/Pax-6 gene was responsible for initiating the development of light sensitive cells in early metazoans and has remained largely unchanged even as eyes have evolved into much more complex and different structures.
Hox genes in Vertebrates
Vertebrates have essentially the same set of Hox genes as do arthropods, but in vertebrates there are four sets of these genes (instead of only one as in arthropods).
Cephalochordates (which include the Lancet) and Tunicates, both invertebrate chordates, have only one set.
This (along with data from some other well conserved genes) suggests that two rounds of whole genome duplication must have occurred near the base of the vertebrate clade.
Here is a phylogeny of the phylum Chordata.
As in arthropods, the vertebrate hox genes are expressed in order, anterior to posterior.
In addition, the vertebrate hox genes are expressed in a temporal order, with those at the 3' end expressed
earlier (as well as more anteriorly) than those towards the 5' end.
Though vertebrate Hox genes appear to provide positional information, mutations in them do not produce
the switching of body parts that we see in arthropods.
Quadrupling the number of Hox genes may have facilitated increased structural complexity in
Vertebrates.
Plants
Plants have a similar set of genes, that are expressed in particular
patterns and regulate the expression of other genes.
These are not, however, homeobox containing genes.
Instead, they contain a region known as the MADS box - which is not
homologous to the homeobox.
For example, in Angeosperms, the identity of floral parts is determined by a set of MADS genes designated A, B, and C.
Flowers are composed of four basic types of structures, arranged in whorls:
Sepals, Petals, Stamens, and Carpels.
Overlapping expression of transcription factors determines the identity of these elements:
A - Sepal
AB - Petal
BC - Stamen
C - Carpel
If B is knocked out, then petals become sepals and stamens become carpels.
Interestingly, animals have some MADS box genes and plants have some homeobox containing genes, however, neither animal MADS genes nor plant homeobox genes appear to play a major role in pattern formation.
This is consistent with the fact that multicellularity, and the developmental processes underlying it, evolved independently in animals and plants.
From an evolutionary standpoint, organisms are lifecycles.
We measure fitness as the number of descendants that an individual leaves after one entire turn of the cycle.
Thus, each part of the cycle contributes to fitness.
Examples of different life history strategies:
| Spread out reproduction | vs | Reproduce all at once |
| Reproduce early and die young | vs | Live a long time and delay reproduction |
| Produce many offspring each with
low chance of survival | vs | Produce few offspring with high chance of survival |
Which of these strategies evolves is determined by adult survivorship and by fertility as a function of age.
Nomenclature
lx = Probability of surviving to age x
px = Probability of surviving from age x to age x+1.
mx = Expected number of offspring produced at age x