Evolution, 4th Edition

350 CHAPTER 14

Gene duplication happens on a massive scale when mutation produces a tetra-

ploid (see Chapter 4). A tetraploid descendant of a diploid has four copies of each

gene rather than two. Tetraploids can arise in two ways (see Chapter 9). The first

is by whole genome duplication. This can occur when the genome of a single spe-

cies is doubled (resulting in an autopolyploid). Whole genome duplication can also

occur when two species hybridize, and the gametes from both of them are mutants

with unreduced diploid genotypes (resulting in an allopolyploid). Further rounds of

hybridization can give rise to species with six, eight, and even more copies of each

chromosome. Whole genome duplication is much more common in plants than

animals, but it did occur twice in our own remote ancestors, between 650 and 550

Mya [33]. Recent events of polyploidy occurred during the domestication of several

important crop plants (including wheat, coffee, and cotton) and were key to improv-

ing some of their economically valuable traits [51].

While recombination usually involves the mixing of genes of the same species,

occasionally genes from other species are mixed into the gene pool. In eukaryotes,

this usually occurs through hybridization between closely related species. Genetic

exchange also happens between distantly related organisms by horizontal gene

transfer, or HGT (see Chapter 4). HGT is particularly important to prokarytotes,

and is the most common way by which they acquire new genes, including those

that confer antibiotic resistance. HGT can vastly speed up adaptation since a new

functional gene is acquired in one fell swoop, rather than evolving through many

mutations.

HGT has been important in the evolution of the nematodes that are parasites on

plants. The worms invade the roots with the help of cellulases, pectate lysases, and

other enzymes that break down the cell walls of the plant. These enzymes were

acquired by the nematodes from bacteria and fungi [8]. HGT thus opened up an

entirely new ecological niche for nematodes, and now enables them to be major

pests of crops around the world.

Given the intimacy between the genomes of the mitochondria and the nucleus

within a cell, it may not be surprising that extensive HGT has occurred between

them [1]. The vast majority of the transfers have been from the mitochondria to

the nucleus. The movement of genes from the mitochondria to the nucleus has led

to large reductions in the size of the mitochondrial genome. In the most extreme

cases, all mitochondrial genes have moved to the nucleus. Mitochondria have lost

their entire genomes in several unicellular eukaryotes (such as the microsporidia, a

group of intracellular parasites).

It might seem impossible that a new gene could originate from DNA that previ-

ously had no function. After all, the number of possible combinations of codons is

beyond comprehension, and the chance that a random combination might make a

Futuyma Kirkpatrick Evolution, 4e

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(A) Adh

(B)

Adh gene retrotransposes

into intron 3 of the

Ymp gene

New stop codon

jingwei from the Adh gene

Ymp

Exons

degenerate

FIGURE 14.8 A new Drosophila gene called jingwei origi-

nated by the retrotransposition of one gene into the intron

of another. (A) The Adh gene (with exons shown in purple)

was retrotransposed into the third intron of the Ymp gene

(with exons shown in green) about 2 Mya. (B) After the ret-

rotransposition event, the exons downstream of the novel

exon degenerated because of the addition of the new stop

codon at the end of the Adh sequence (red bar). The name

jingwei comes from a Chinese myth in which a princess

metamorphoses into a new form. (After [42].)

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