The Scientist November 2019

dria are functionally divergent in ways

that are clearly adapted to different water

temperatures.^5 In spite of this, there does

not appear to be evidence of mitonuclear

incompatibilities between northern and

southern populations—studies show the

two populations can mate without any

untoward consequences.^6

One of the challenges in studying

mitonuclear interactions in the context

of speciation is that it’s difficult to estab-

lish a causal relationship: it is possible

that mitonuclear mismatches could arise

due to drift after a speciation event, rather

than drive the reproductive isolation in

the first place. These are questions that

Michi Tobler, a biologist at Kansas State

University, is trying to answer. His study

system of choice: the fishes of the family

Poeciliidae, several lineages of which have

adapted to live in hydrogen sulfide–rich

springs in Mexico.

Hydrogen sulfide is extremely toxic

because it binds to and blocks cytochrome

c oxidase, thus interrupting mitochondrial

function. Tobler wanted to understand how

the fish could survive in these conditions, and

to figure out whether mitochondrial adapta-

tions to sulfidic environments could cause

mitonuclear incompatibilities that would

promote speciation. He has discovered that

in some lineages, one of the mitochondrial

proteins in the cytochrome c oxidase enzyme

has evolved a shape that cannot be inhibited

by hydrogen sulfide.^7 The modified protein,

along with evolved changes in gene expres-

sion, allows the fish to survive the toxic envi-

ronment. This should, in theory, exert selec-

tive pressure for compensatory changes in

the nuclear components of cytochrome c

oxidase—something he is currently trying

to suss out across lineages that have been

separated from their cousins in non-sulfidic

waters for various lengths of time.

This is an ideal study system, he says,

because “we basically get snapshots at dif-

ferent time points during speciation,” reveal-

ing clues as to how the animals evolved.

Asymmetrical inheritance

The fish adapting to sulfidic springs are

a clear example of mutations in the mito-

chondrial genome leading to changes in

CONSEQUENCES OF
MITONUCLEAR INTERACTIONS
The intimate relationship between the mitochondrial and nuclear
genomes comes into play as populations evolve. For example, the rela-
tively fast mutation rate of mitochondrial DNA (mtDNA) means that the
nuclear genome (nDNA) has had to evolve compensatory mutations to
keep pace and maintain collaborative functionality. This process causes
populations to drift apart due to mitonuclear incompatibilities.
Copepods on the Pacifi c coast of North America are the best-known
example of this phenomenon. Researchers have successfully bred animals
from diff erent tide pools, and while the fi rst-generation hybrids do fi ne,
second-generation individuals develop slower and have fewer off spring.

LUCY

READING-IKKANDA

Santa Cruz

male

Male

F0

Female

F2

Male

F0

Male

F0

Male

F0

Female

F2

Female

F2

Female

F2

nDNA Parental

generation

(F0)

First-

generation

hybrid (F1)

Second-

generation

hybrid (F2)

REDUCED

FITNESS

mtDNA

Santa Cruz

male

California, USA

Santa

Cruz

San Diego

Santa Cruz

female

Santa Cruz

female

MITOCHONDRIAL MATCHES

When F2 hybrids are backcrossed to the paternal line, they show no improvement in fi t-

ness. When they are backcrossed to their maternal line, however, their fi tness is rescued,

most likely because the backcross in this direction reintroduces the nuclear genome to the

mitochondrial background it is co-adapted with.

F2 hybrid females crossed with

paternal line, where mitochondria

types do not match, leads to no

fi tness improvement.

F2 hybrid females crossed with

maternal line, which carries

the same mitochondrial type,

improves fi tness.