11.2019 | THE SCIENTIST 43© SCIENCE PHOTO
LIBRARY, SCIEPRO
F
rom Alaska down to the Baja Pen-
insula, the rocky tide pools of North
America’s West Coast are sepa-
rated by hundreds of kilometers of sandy
beaches. Inside those tide pools live Tigri-
opus californicus copepods, small shrimp-
like animals that evolutionary biologist
Ron Burton has been studying since he
was an undergraduate at Stanford Univer-
sity in the 1970s. During those early days
of DNA technology, Burton became curi-
ous how the genomes of the isolated cope-
pod populations compared.
While still at Stanford, Burton
sequenced the mitochondrial gene cyto-
chrome c oxidase subunit one, the standard
marker people used at the time for spe-cies identification, and discovered that the
copepod populations were strongly differ-
entiated: on average, there was a 20 percent
sequence divergence in this gene between
populations. When he crossed Santa Cruz
copepods with animals from San Diego, the
hybrids did fine, but when he bred them to
one another, their offspring did not do well,
taking longer to develop, producing fewer
offspring, and having lower survival.^1 “That
was the first indication that there was some
sort of genetic incompatibility developing
between these isolated populations,” says
Burton, now a professor at the Scripps
Institution of Oceanography in San Diego.
The first-generation hybrids have a full
set of nuclear genes from each parent, butamong their progeny, genetic recombina-
tion has mixed and matched the paren-
tal genomes. This potentially creates
mismatches between the nuclear and
mitochondrial genomes that affect fitness.
When Burton mated the second-generation
female hybrids with male copepods from
the parental Santa Cruz or San Diego pop-
ulations, he found that the direction of the
backcross made a difference. Breeding the
second generation of hybrids with mem-
bers of the paternal population produced
no improvement in fitness. But a back-
cross to the maternal population produced
offspring that were just as fit as the orig-
inal natural populations. This happened
regardless of whether the maternal popula-A revival of interest in mitochondrial biology is revealing the importance of
their genetic interactions with the nucleus in many facets of physiology, and
even in the evolution of species.BY VIVIANE CALLIER