Biology Reference
In-Depth Information
the emergence of population genetics in the 1930s (Sapp, 1990). During this time, chromosomal
genes quickly replaced symbionts as what were considered the main building blocks of evolutionary
change. The rise of the modern synthesis led to a fall in symbiosis research, and WallinÔs thesis
was ultimately characterized by skeptical remarks, such as Ñit is within the range of possibility that
they (symbionts) may some day call for more serious attentionÒ (Wilson, 1925).
Some 75 years later, WallinÔs day may have Ýnally arrived. The last decade has brought renewed
interest and even some agreement on the role that endosymbionts play in evolutionary processes.
There are clear footprints of endosymbiosis in major evolutionary transitions (Gray et al., 1999)
as well as microevolutionary processes including adaptation and speciation (Margulis and Fester,
1991; Douglas, 1998). Although endosymbionts are certainly not the major causal factor in species
formation, as Wallin argued, they are now viewed as at least one of the factors that can promote
the speciation process. Nowhere is this more clear than in studies of
-
proteobacteria that are among the most abundant endosymbionts in the world. Werren (1998) last
reviewed the topic, but signiÝcant advances have been made since then that have shifted the
controversy over
Wolbachia
, a group of
b
-associated speciation from whether it is plausible to how important it is.
Inferences about how symbionts can generate evolutionary novelty and new species have lurked
in the background of speciation research for quite some time (Hoyt and Osborne, 1971; Williamson
et al., 1971; Howard et al., 1985; Nardon and Grenier, 1991; Margulis, 1993; Adams and Douglas,
1997). Indeed, there is an immense body of work on the host cellular, biochemical, developmental,
and evolutionary changes caused by symbiotic miroorganisms (Margulis and Fester, 1991;
Chapter
1,
this topic). But these studies are rarely taken as serious evidence of symbiont-induced speciation
because it is unclear which and how often these intraspecies changes affect the process of speciation.
In this regard, the cytoplasmically inherited bacterium
Wolbachia
has emerged as the poster child
for studies of speciation by means of symbionts. The reason is simple Ð these bacteria alter
reproduction and therefore can directly affect compatibility between populations or species.
My objectives here are to brieÞy introduce the reader to
Wolbachia
-associated speciation and
to highlight the advances that have been made in the Ýeld since the last review of the topic. I will
also evaluate the criticisms of
Wolbachia
-associated speciation and emphasize those areas where
more research is badly needed. Finally, I will discuss some alternative systems in which symbionts
may drive host speciation. Throughout this chapter, it is important to keep in mind that more than
60 years of speciation research have fortiÝed the nuclear gene as a dominant agent of species
formation (Coyne and Orr, 1998); but it is this very fact that makes the possibility of symbiont-
induced speciation still so Ñstartling.Ò
Wolbachia
DEFINING A SPECIES AND SYMBIONT
DeÝning a species is a controversial issue (Harrison, 1998; Hey, 2001; Noor, 2002), especially
when considering a symbiotic basis of speciation. I will avoid lengthy discussions on this topic,
as this is not the focus of the chapter. I simply adhere to the widely accepted Biological Species
Concept (Mayr, 1963), in which species are Ñreproductively isolatedÒ groups comprised of
potentially interbreeding individuals. Reproductive isolation simply refers to those mechanisms
that prevent or reduce interbreeding between such groups. There are two general forms of
reproductive isolation: (1) postmating isolation refers to those mechanisms that hinder the Þow
of genes after mating takes places (e.g., hybrid sterility or inviability), and (2) premating isolation
refers to those mechanisms that affect interbreeding before mating takes places (e.g., mate
discrimination, habitat differences). Both pre- and postmating isolation ultimately reduce gene
exchange between groups and therefore allow different species or diverging populations to evolve
independently of each other. By adhering to this species concept, speciation can be equated to
the evolution of reproductive isolation.
A symbiont is one of the organisms involved in an intimate association between two organisms.
Symbionts are sometimes strictly considered to be mutualists, but I will use the more general and
