Evolution__3rd_Edition

624 PART 5 / Macroevolution
Box 22.1
The Evolutionary Genomics of Fruit
Many plants produce large, nutritious fruits. Fruits probably
evolved to attract birds and mammals, which disperse the plant
seeds when they eat the fruit. Specialized seed dispersal relations
with vertebrates are another coevolutionary mechanism by which
angiosperm diversity may have been increased. However, once
fruits had evolved, several other taxa evolved to exploit them,
though not in ways that benefit plants. Fungi digest fruits, making
them rot. Some insects, such as fruitflies, lay eggs on or in the fruit,
and their larvae eat the fruit from within.
Fossil evidence of fruits is found from the early Tertiary, around
60 million years ago. The evolution of fruit manufacture in plants,
and of fruit exploitation in vertebrates, insects, and fungi, required
special genes coding for appropriate developmental and metabolic
circuits. Yeast, for instance, have a special metabolic circuit for fruit
digestion a the circuit that produces alcohol as a by-product and is
the basis of beer, wine, and other drinks. The genes that code for
this circuit have been identified in the yeast genome. The genes
originated by duplications, and the time when that happened can
be dated by the molecular clock (Section 19.3, p. 559) to about
80 million years ago. This date is somewhat earlier than the fossil
date for the first fruit. Maybe the fossil date is too late, or the
molecular clock is unreliable.
What about fruit-consuming insects? The Drosophilidae are a
family of fruit exploiters. They originated about 65 million years
ago, and probably evolved fruit-exploiting metabolism around that
time. It would be interesting to date the duplications that generated
fruitfly alcohol dehydrogenase (Sections 4.5, p. 83, and 7.8.1,
Parasite–host coevolution is
antagonistic
p. 180) and other fruit-related enzymes. Maybe they would date
to around this time too. (Incidentally, fruitfly alcohol dehydrogenase
is unrelated to the enzyme of the same name in humans.) Further
evidence may also come from the genomes of angiosperms. If we
could identify genes used in fruit manufacture, we could date
them and see whether they also originated around this time.
Alternatively, we could do some evolutionarily inspired gene
hunting. In the Arabidopsis genome, many duplications have been
dated, but the functions of many of the duplicated genes remain
unknown. Vision et al. (2000) published a picture of the time course
of duplication events in the history of Arabidopsis. The picture has
a minipeak around 75–80 million years ago. Maybe some of the
genes in that minipeak will be related to fruit manufacture.
(Arabidopsis itself does not produce fruits, but its genome could
contain the ghosts or relatives of fruit genes.)
In conclusion, the origin of fruit led to evolutionary changes
in several taxa a fungi, insects, and vertebrates. These changes
occurred independently in each of the taxa. Genomic evidence
can be used to date events in the past, once we have identified the
genes concerned. We can predict that the evolutionary changes in
all taxa should have occurred at much the same time, or at least
that the changes in fungi and animals should follow the changes
in plants. The evidence so far is incomplete, but tantalizing. It
also illustrates how genomics is being integrated with existing
paleontological and ecological methods in the study of coevolution.
Further reading: Ashburner (1998), Dilcher (2000), Benner et al. (2002).
selection for a change in the host. If the range of genetic variants in parasite and host is
limited, coevolution can be cyclic (Section 12.2.3, p. 325); but if new mutants continually
arise, the parasite and host may undergo unending coupled changes that may or
may not be directional according to the type of mutations that arise. Coevolution in
parasites and hosts is antagonistic, unlike the mutualistic coevolution of ants and caterpillars
or of flowering plants and pollinators.
Many biological properties of parasites and hosts have been attributed to coevolution.
Here we concentrate on two. The first is parasitic virulence. In informal terms,
virulence means how destructive the parasite is. In formal terms, virulence is expressed
as the reduction in fitness of a parasitized host relative to an unparasitized host. A
highly virulent parasite is one that kills its host quickly, reducing the host’s fitness to
zero. The virulence of a parasite is normally thought to be a side effect of the manner
in which the parasite lives off its host. If, for instance, a parasite consumes a large
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