BIL 104 - Lecture 15

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PHENOTYPIC EFFECTS OF ABNORMAL CHROMOSOME NUMBER, CONTINUED

First, a look at

SYNTENY (from the Greek syn, "same" and tene, "thread")

Simply defined, SYNTENY is the presence in two different species of segments of DNA with the same gene sequences.

Synteny provides evidence that multiple translocations and chromosomal rearrangements of originally similar chromosomes has been an important feature of evolutionary change.

Example: human vs. mouse genome:

MUTATIONS/CHANGES AT THE LEVEL OF THE ENTIRE CHROMOSOME SET

One chromosome set: HAPLOIDY

Two chromosome sets: DIPLOIDY

Multiple chromosome sets: POLYPLOIDY (You can specify how many sets!)

In most animals, polyploidy is lethal (it is known in a few reptiles, fish and invertebrates, but it's always lethal in mammals and birds, as far as we know). Haploidy is normal in some species, such as honeybees (drones), but it is not common.

In plants, polyploidy is an important mechanism for speciation.

ALLOPATRIC speciation can occur when a formerly contiguous population becomes divided by a geographic or other impassable barrier. Over time, genetic drift, mutation, natural selection and other factors can result in the reproductive isolation of the separated parts of the original population, creating two new species.
SYMPATRIC speciation occurs when a subset of a population becomes reproductively isolated from the larger population, WITHOUT geographic separation.
In some cases, a genetic event can cause the instantaneous reproductive isolation of part of a population. This type of sympatric speciation is sometimes called STASIPATRIC speciation. In plants, this can occur due to polyploidy (the new polyploids can't back cross with the original, parental population).
Three general ways this can happen:
1. Autopolyploidy (all chromosome sets from the same species)
2. Allopolyploidy(chromosome sets from different species) - two forms of this type.
Note: usually, a polyploid plant is much bigger, more robust and healthy than its parental diploid stock!

Fun with Polyploidy

Some species are sufficiently closely related that their genes, when combined in a hybrid individual, provide the necessary information for a viable organism--but not for that organism to undergo normal meiosis.

Homo sapiens 2n - 46

Pan troglodytes 2n - 44

Hybrid between the two would be 2n = 45.

(BUT THIS HAS NOT HAPPENED, DESPITE WHAT THE WEEKLY WORLD NEWS WILL TELL YOU.)

In some cases, closely related species *can* hybridize, and if their chromosome sets differ in number, each chromosome in the hybrid is univalent (i.e., it has no homologous "mate"), and meiosis cannot proceed normally (no synapsis; no crossing over; random segregation of c'somes into forming gametes; inviable gametes)

Viable hybrids have been produced in the laboratory, and it's possible that they could occur in similar fashion in natural situations. Here's an example of an artificially produced "new species." (Well, sort of.)

Brassica oleracea (cabbage) x Raphanus sativa (radish)

Both species 2n = 18; n = 9
Breeding between the two species produces an F1 generation in which 2n = 18 (but actually is 9 + 9, since the chromosomes are not homologous).
But if somatic doubling (i.e. nondisjunction of an entire set!) occurs in a MERISTEMATIC cell that later develops into a reproductive part of a flower...
Result: 2n = 36. In effect, each parental chromosome set has "created" its own homologous set and migrated with it into a new cell.
At this point, normal meiosis, complete with synapsis and crossing over can occur. Because these plants are usually self-fertile, they can produce offspring, even if there is only one such individual. This "new genus/species" was named Raphanobrassica.
Backtracking through natural matings of other species, one can see how new species may be produced via the non-finicky mating habits of some plant species.

Animals can also produce hybrids (e.g. horse x donkey yields a mule; lion x tiger yields a liger or tigon), but these are almost invariably sterile.

animals have chromosomally determined sex, and polyploidy interferes with this.

animals have multiple biological isolating mechanisms (geographic, temporal, behavioral etc.) which tend to prevent natural interbreeding between species.

plants retain meristematic (embryonic) tissue throughout their lives and are self-fertile: these conditions are favorable for successful allopolyploidy.

ANEUPLOIDY VS. POLYPLOIDY: Gene Balance

Note that in almost all cases, aneuploidy is much worse for the organism that has it than polyploidy.

Also, monosomies (haplo-abnormal) are much worse than trisomies (triplo-abnormal).

This is probably due to GENE BALANCE, which means that proper development depends not only on the absolute quantity of transcripts of a particular gene, but also on the RATIO of that gene product to the other gene products produced in a normal cell.

Presumably, the amount of transcript from a given gene is directly proportional to the number of copies of that gene in the cell. This means...

In a polyploid individual, gene products should still be present in the same ratios as found in a normal, diploid cell.

In an aneuploid individual, however, the PROPORTIONS of gene products will be different from those found in a normal, non-aneuploid individual.

The delicate balance and interaction between gene products, when changed, results in mild to severe abnormalities in aneuploid individuals.

This will probably make more sense when we discuss DEVELOPMENTAL GENETICS, since it is here that we see how relative proportions of gene products has a profound effect on determining the direction of developmental "cascades."

Note that X-chromosome inactivation (dosage compensation) is related to this phenomenon. The Y chromosome is believed to be a degenerate X chromosome (far back in evolutionary history...). In a sense, it can almost be said that the normal condition for the X chromosome is MONOSOMIC, except that there are about 18 genes that are operational even on the Barr Body.