Biology Reference
In-Depth Information
FROM WALLIN TO
WOLBACHIA
AND BEYOND
Debates about the types of heritable elements that promote speciation began as early as the 1920s
and 1930s among evolutionary biologists and geneticists. The two extremes of the debates are best
represented by two topics that were published within the span of 10 years Ð Ivan WallinÔs
Symbionticism and the Origin of Species
(1927) and Theodosius DobzhanskyÔs
Genetics and the
Origin of Species
(1937). Despite WallinÔs effort to put symbiosis into the mainstream of evolu-
tionary biology, only DobzhanskyÔs book would have a lasting inÞuence through the century. From
the Biological Species Concept to the DobzhanskyÏMuller model of postzygotic isolation,
Dobzhansky laid a solid foundation for future study of the genetics of speciation (Orr, 1996).
But now with the progress in the
Wolbachia
Ýeld and the advent of molecular biology techniques
that make the identiÝcation of bacterial endosymbionts simple, perhaps a reassessment of WallinÔs
ideas on the symbiotic origin of species is needed. The emergence of
Wolbachia
in topical discus-
sions of invertebrate speciation is just a start to reviving WallinÔs silenced ideas on the role of
endosymbionts in speciation. A full treatment of their role will go beyond
Wolbachia
and reveal
the diverse ways in which other symbionts contribute to speciation. In closing, I brieÞy mention a
few of these alternative systems below.
Many organisms complete their entire life cycle on a single host, and thus divergence in host
speciÝcity may be an important engine of speciation (Bush, 1994). Analyses of reproductive isolation
associated with host shifts have focused largely on the nuclear-genetic basis of host speciÝcity
(Hawthorne and Via, 2001). However, there is ever increasing evidence that, in addition to nuclear
genes, endosymbionts can also play a crucial role in host nutrition and adaptive radiations onto new
resources (Margulis and Fester, 1991). For example, the weevil genus
Sitophilus
is the only genus
of the Rhynochophorinae family that lives on cereal grains; the other genera live at the junction of
the roots and stems of monocotyledons (Nardon and Grenier, 1991). This difference in host speciÝcity
is due to the nutrients provided by cytoplasmic bacteria that infect these weevils.
Buchnera
endo-
symbionts are nutritionally important to more than 4400 aphid species, many of which occur
sympatrically and differ in host use (Guildemond and Mackenzie, 1994; Adams and Douglas, 1997).
Perhaps divergence in
Buchnera
can also drive divergence in aphid plant use. Even host shifts in
the
Rhagoletis
fruit Þy genus may be inÞuenced by their predominant symbiotic bacteria,
Klebsiella
oxytoca
(Girolami, 1973; Howard et al., 1985). These kinds of symbiont-based adaptations warrant
further investigation of their role in ecological shifts and host race/species formation.
Sex attractants or pheromones can also be key players in the evolution of premating isolation
and speciation (Cobb and Jallon, 1990; Coyne et al., 1994). What might surprise some is the fact
that symbiotic bacteria can secrete products that act as sex attractants. Males of the grass grub
beetle
Costelytra zealandica
are attracted to products generated from bacteria located in the
colleterial glands of this species (Hoyt and Osborne, 1971). These glands are represented as an
outpocketing of the vagina and are well supplied with tracheae. Microbially induced sex attractants
have been generally ignored but may prove signiÝcant in speciation given the extensive distribution
of normal bacterial Þora in animals.
Finally, cytoplasmic bacteria have also been implicated in more classic isolating mechanisms
including hybrid male sterility in
Drosophila
species (Williamson et al., 1971; Powell, 1982) and
in
Heliothis
moths (Krueger et al., 1993).
The diversity of host organisms and traits affected by symbionts suggests that our awareness
of symbiont-induced speciation may be limited. While
Wolbachia
currently stand out as a model
system for studies of symbiont-induced speciation, there is the more exciting prospect that in the
near future we will have a collection of diverse bacteria that can act as species splitters.
ACKNOWLEDGMENTS
I am grateful to John Jaenike, Daven Presgraves, DeWayne Shoemaker, and Jack Werren, whose
suggestions, corrections, and discussions greatly improved this chapter. I wrote this work during
