Molino and Tinaja fish are both much
reduced in pigmentation compared to the
surface fish from which they descended.
Both cave fish have greatly reduced
numbers of melanin cells compared to
those in the surface fish, and the Molino
fish completely lacks melanin and is an
albino. In the hybrid between the Tinaja
and Molino fish, however, there is almost
a complete restoration of the normal
pigmentation seen in the surface fish. Even
the black stripe on the tail is restored!
Populations of fish established in caves
evolve toward pigmentation reduction
through the accumulation of mutations
in genes that are necessary to develop and
maintain the pigment cells. Because the
two populations were isolated from each
other, the actual mutations in the genes
were different from one another.
When the two parents are mated, the
normal and the mutated genes are both
inherited by their offspring and the normal
ones have the opportunity to mask the
effects of the mutated ones. That is, for
every mutated gene copy transmitted to the
hybrid from the Molino parent, there would
be a good copy coming from the Tinaja
parent, and vice versa. Since even one
good copy of a gene would be enough to
restore some function, the hybrid has more
pigment cells than either of its parents.
Such genetic complementation with
two different parental mutations is a well-known phenomenon, even in humans.
For example, when two deaf parents have
a child, sometimes the child’s hearing is
The eye rudiments of the cave hybrids
are larger than those in either the Molino
or Tinaja parent populations, but the
restoration is not as dramatic as for
By making hybrid crosses, Wilkens
estimated that small eyes were due to
mutations in about 10 different genes in
each population. We later confirmed his
estimate by directly mapping the genome
of the cave fishes.
The question then is whether the
mutated genes are the same in every
population. If the two cave populations
were geographically close, there might be
some mating exchange between them and
some of the mutated eye genes might
be the same in both populations, but if
they were distant and no mating occurs
between them, it is possible that none of
the mutated eye genes would be the same.
Hybrids between cave populations saw a return of pigmentation and eye development.
Take a hypothetical example of two cave
populations that are close enough to one
another that some mating occurs and some
of the mutated genes are shared by both.
If the two populations were so far apart
that none of the mutated genes are shared
between populations, then the hybrid
would receive one good copy of each gene
from one or the other parent. In this case,
the eyes of the hybrids will be even larger
than in the first case, perhaps even normal.
If you want to see the full effects of
inter-cave hybridization on eye size, you
have to look at very young fish. One
example showed that the eye rudiments
of Tinaja and Molino cave fish are much
smaller than those of surface fish. Not
only are they smaller, they are completely
nonfunctional. In contrast, the eye
rudiments of the Tinaja x Molino hybrids
are significantly larger than those of their
parents. In order to determine whether
any of these fry could see, we placed them
individually in a shallow dish in a syruplike substance that made it difficult for
them to swim but did not interfere with
their breathing or eye movements. The
dish was surrounded by a vertical cylinder
with black and white vertical stripes that
we could rotate.
When we tested the surface fish, their
eyes followed the stripes as they rotated
and then, going as far as they could,
they would snap back. This happened
repeatedly, until we stopped the movement
or reversed the direction of rotation. When
we reversed the direction, they would
follow in the new direction and snap
back in the opposite. The surface fry did
this because they could see and because
following stripes is an instinct.
Tropical Fish Hobbyist www.tfhmagazine.com