Evolution__3rd_Edition

..
Following symbiosis ...
. . . genes have transferred to the
nucleus
CHAPTER 19 / Evolutionary Genomics 563
any case, we need to understand the rates of gene gain and loss in order to understand
the sizes of the genomes (and of different parts of the genomes) in different species.
19.5 Symbiotic mergers, and horizontal gene transfer,
between species influence genome evolution
Most genome size increases in evolutionary history have occurred by duplications, of
all or part of the genome. But duplications are not the only mechanism. Two species
may also combine their genomes into one (or almost one), in a particularly intimate
symbiosis that is rather like a business merger. In the history of human DNA, only one
such event is known: the symbiosis between two bacteria that led to the eukaryotic cell
containing a mitochondrion (Sections 10.4.3, p. 265, and 18.3.2, p. 533). The event probably
took place 2,000–2,500 million years ago. The genome sizes of the two bacteria
concerned is unknown. However, modern bacteria have a range of gene numbers, from
less than 1,000 to over 6,000, with an approximate average of about 2,500 genes. The newly
merged cell might have had two DNA molecules, each containing about 2,500 genes.
Since that time, one of the DNA molecules has expanded and evolved into the
nuclear DNA while the other has shrunk and evolved into the mitochondrial DNA. All
modern animals have mitochondria of about the same size, containing 13 proteincoding
genes and 24 RNA-coding genes. The mitochondria of plants and microbes
show a greater range of genome sizes, some being larger and others smaller than in
animals; but even the largest mitochondrial genomes have only 100–200 genes. The
reduction in gene numbers has mainly been by gene loss, in the same manner as in
other bacterial intracellular symbionts (see Section 19.4). The genes were unnecessary
after the symbiosis and were lost. But some mitochondrial genes were transferred to the
nucleus. The nuclear DNA of modern human beings contains genes descended from
both of the original eukaryotic merger partners. The process of gene transfer from
mitochondria to nucleus is difficult to study in animals, because the mitochondrial
genome is relatively constant. However, in plants, genes seem to be transferred more
frequently and some revealing research has been done.
For example, in many plants the gene coding for ribosomal protein S14 (rps 14) is in
the mitochondrion (Kubo et al. 1999). But in rice the rps 14 gene in the mitochondrial
genome is dysfunctional (it is a pseudogene). Instead the rps 14 gene is found in the
nucleus. The gene is found in an interesting place. It is inside an intron, which in turn is
inside the gene for mitochondrial succinate dehydrogenase (sdhb). sdhb is an earlier
mitochondrial gene that moved to the nucleus and codes for a protein that operates in
the mitochondrion. One problem faced by a mitochondrial gene that is accidentally
copied into the nucleus is that special targeting signals are needed for a protein to enter
the mitochondrion. The mitchondrial gene must somehow acquire the targeting signal
sequences if it is to work from the nucleus. The rps 14 gene, by entering the sdhb gene,
neatly solved this problem. Ribosomal protein S14 and succinate dehydrogenase are
generated by alternative splicing (Section 2.2, p. 24) from the compound gene.
The rps 14 story illustrates how genes transfer from the mitochondrion to the
nucleus. The “targeting signal” problem is one of several detailed problems that have to