Biology Reference
In-Depth Information
historical view that symbionts can take on any relationship with the other organism, including
parasitism and commensalism (de Bary, 1879). The text will focus on endosymbionts Ð those
symbionts that exist inside (e.g., intracellularly) their hosts.
WOLBACHIA
AS A MODEL SYSTEM FOR
SYMBIONT-INDUCED SPECIATION
Before considering the advances in the frontier of
Wolbachia
-associated speciation, it is important
. These features, which have been discov-
ered only within the last decade, form the conceptual landscape for why
to summarize some key biological features of
Wolbachia
stand apart
from other symbionts implicated in promoting speciation (Margulis and Fester, 1991). For those
readers unfamiliar with
Wolbachia
biology, this section will also serve as a brief introduction to
this fascinating bacterium. There are four important features:
Wolbachia
A
BUNDANCE
Wolbachia
are among the most abundant endosymbiotic bacteria on the planet, due in part to their
unparalleled host range. First discovered in the mosquito
Culex pipiens
(Hertig and Wolbach, 1924),
are estimated to occur in 20 to 75% of all insect species (Werren et al., 1995a;
Jeyaprakash and Hoy, 2000), 35% of terrestrial isopods (Bouchon et al., 1998), 43% of mites
(Breeuwer and Jacobs, 1996), and almost all Ýlarial nematodes (Bandi et al., 2001). Thus,
Wolbachia
Wolbachia
infect at least two animal phyla (Arthropoda and Nematoda) and are at high frequencies within
two of the most speciose groups of animals Ð insects and mites. Extrapolating these various
infection frequencies to the estimated number of species in these taxa places
in several
million host species. These numbers speak for themselves and have obvious implications for the
potential importance of these symbionts in host speciation. Limits to the host range (e.g., vertebrates
or other invertebrate groups) are currently not known.
There are at least four major subgroups of
Wolbachia
, labeled A through D. Subgroups A and
B diverged ~60 million years ago (Werren et al., 1995b) and occur strictly in arthropods. Subgroups
C and D are speciÝc to Ýlarial nematodes and diverged from the common ancestor of A and B
~100 million years ago (Bandi et al., 1998). Arthropod species can be either singly or multiply
infected with A and B
Wolbachia
(Werren et al., 1995a). Based on phylogenetic work and the
occurrence of double infections, it is inferred that horizontal transmission of the A and B
Wolbachia
Wolbachia
must occur at some level (OÔNeill et al., 1992; Werren et al., 1995a; Stouthamer et al., 1999).
R
A
EPRODUCTIVE
LTERATIONS
Unlike other symbionts that spread through host populations by enhancing the Ýtness of their host,
Wolbachia
parasitize host
reproductive strategies in four basic ways Ð male killing, feminization, parthenogenesis, and
cytoplasmic incompatibility (CI) (Werren, 1997). Because these bacteria are inherited through egg
cytoplasm, they are selected to increase the number of infected females (i.e., the transmitting sex)
in a population, even at the expense of males. Such examples illustrate the ongoing cytonuclear
conÞict over sex determination and sex ratios, which can in turn play an important role in rapid
evolutionary changes and subsequent genetic divergence among populations.
BrieÞy, male killing occurs when infected male embryos die such as in the ladybird
can spread by reducing the Ýtness of their host. In arthropods,
Wolbachia
Adalia
bipunctata
(Hurst et al., 1999; Hurst et al.,
2000). This effect imparts a Ýtness advantage to infected female siblings, perhaps through reducing
the Ýtness cost of competition with siblings. Feminization occurs when infected genetic males are
converted to phenotypic females who are able to transmit the bacteria (Rousset et al., 1992). Parthe-
nogenesis induction (PI) typically occurs in haplodiploid wasps in which infected virgin females
, the butterÞy
Acraea encedon
, and
Drosophila bifasciata
