1.3.1. Characteristics and incidence of male killers
Embryonic male killing symbionts come from a diverse range of bacterial lineages separated by at least 2, 000 million years (Hurst et a i, 1997a). There are no examples of bacterial clades that contain solely male killing lineages. In all cases, male killing is believed to be a derived state, evolving in bacteria that already show maternal
inheritance (Schulenburg et al., 2000). Male killers are known to infect an ever-growing multitude of arthropod hosts (Table 1.3.). In most of the cases illustrated in the table, the systematic affiliations derive from 16S rDNA sequence data from the bacteria associated with the trait. Subsequent polymerase chain reaction (PGR) of infected and uninfected specimens confirms the association of trait and bacterium. Because males are killed at embryogenesis, a low (approximately half) host egg hatch rate is a characteristic of infection, in addition to the female-biased sex ratio. Like all reproductive parasites, male killers are maternally inherited, so breeding subsequent generations of infected hosts
Chapter 1: Introduction 33
where adult sex ratios have been obtained. However, these cases all require molecular systematic analysis of the agent causing the putative male killing (Hurst & Majerus, 1993).
Table 1.3. Biodiversity of early male killers (all bacteria) with examples of a host species from each of the orders infected by a male killing symbiont in each clade
Bacterial Clade Host Reference
Wolbachia Coleoptera Adalia bipunctata Tribolium madens Lepidoptera Acraea encedon Acraea encedana Diptera Drosophila bifasciata
Hurst et al., 1999a Fialho & Stevens, 2000 Jiggins et al., 1998 Jiggins et a i, 2000a Hurst et al., 2000 Rickettsia Coleoptera Adalia bipunctata Adalia decempunctata Hurst et al., 1994 Schulenburg et a i, 2001
Brachys tessellatus Lawson et a l, 2001 Unnamed Flavobacteria Coleoptera
Coleomegilla maculata Adonia variegata
Hurst e/û/., 1997b Hurst et a l, 1999b
Arsenophonus nasoniae Hvmenoptera
Nasonia vitripennis Werren et a l, 1986
Spiroplasma Coleoptera Adalia bipunctata Harmonia axyridis Lepidoptera Danaus chrysippus Diptera Drosophila willistoni Hurst et a l, 1999c Majerus et a l, 1999 Jiggins et a l, 2000b
Chapter 1 : Introduction 3 4 Clearly some male killers are present in many different host groups. What is unclear, as yet, is whether some strains are restricted to particular host taxa. Restrictions might arise following specialisation on a particular host sex determination system: Wolbachia and spiroplasmas maybe the only male killing clades that can easily cross this boundary. 1.3.2. Why kill males?
Sex ratio distorting parasites that cause either féminisation or parthenogenesis induction increase the absolute number of infected female hosts. Male killing appears at first to be more paradoxical because it does not. Whilst males cannot transmit the infection, male ' death is only adaptive if there is a resultant increase in female host survival. It4s
straightforward to visualise this, due to the cytoplasmic inheritance of these pathogeiis. Several factors are believed to affect the spread of a male killer within a host population, by conferring indirect benefits on female hosts; these benefits include:
1. Reduced sibling competition 2. Sibling egg cannibalism 3. Prevention of inbreeding
1.3.2.1. Sibling competition and sibling cannibalism
Either antagonistic actions between siblings, or cannibalism of unhatched eggs (or both) are features of the majority of insects that harbour male killing bacteria, and it is in groups that show these patterns of host ecology that male killer incidence is at its highest (Hurst & Majerus, 1993). Most male killer hosts lay their eggs in clutches and the larvae are gregarious (although there are exceptions such as Danaus chryssipus that is infected with a male killer at 40% prevalence, despite the fact that females lay single eggs (Jiggins et a l, 2000b)). For example, m Adalia bipunctata, the two-spot ladybird, eggs
Chapter 1 : Introduction 3 5 expend energy in foraging for (Osawa, 1992). Significant mortality of neonates that fail to obtain their first meal has been demonstrated in this species (Hurst et a l, 1997a). Intuitively, the intensity of sibling competition in gregarious species is also reduced in infected lines because of the death of approximately half of the clutch.
13.2.2. Inbreeding avoidance
For male killing to give the infected host an advantage through inbreeding avoidance the host must both naturally inbreed and exhibit some level of inbreeding depression
(Werren, 1987). The sole example of an investigation of inbreeding in a male killer infected host comes from A. bipunctata (Hurst et a l, 1996a). In this study, despite the fact that inbreeding depression in the laboratory was found to be severe, the authors found very little evidence suggesting that inbreeding was occurring in the natural
population. They conclude that due to its rarity, inbreeding is unlikely to be important in the dynamics of male killer invasion in the two-spot ladybird. However, it may be true for other host species that inbreeding avoidance plays a large part in symbiont spread. 1.3.3. Direct (physiological) effects of infection with male killing bacteria
The previous section describes indirect effects of infection with a male killer on female host survival. There may also be direct effects of infection on female host fitness. These effects could be negative: the host suffering a physiological cost to harbouring the bacteria; or positive, such as that seen in the A^hiàtBuchnera symbiosis in which the host gains essential nutrients through its obligate association with the symbiont
(Baumann et a l, 1995). To date, all studies looking for direct effects resulting from male killer infection have documented physiological costs to hosts. However, positive effects of infection have been reported in host species infected with other phenotypes of
Chapter 1 : Introduction 3 6 1.3.4. Prevalence variation in n atu ral populations
Huge variation in the prevalence of early male killers has been documented to date ranging from around 1% Drosophila willistoni females infected with Spiroplasma poulsonii (Williamson & Poulson, 1979), to around 90% Acraea encedana females
infected with Wolbachia (Table 1.4.) (Jiggins et a l, 2000a). Whilst an obvious study bias towards highly infected species exists, the facts that many species have early male killers and that they are found in the intensely studied Drosophila, suggests them to be
widespread at low prevalence among the insect groups.
Prevalence of an early male killer within a group is affected by differences in host behaviour, ecology, and interaction with host physiology. Low vertical transmission efficiency and physiological costs to hosts will lead to reduced prevalence of a male killer. However, if the female hosts benefit from the loss of males, as discussed above, the male killer might be expected to reach high prevalence in the population as is seen in some species.
Prevalence can also vary between populations of the same host. For example, different symbiont prevalences are documented in populations that are only kilometres apart, in the butterfly Acraea encedon (Hurst & Jiggins, 2000). Explaining these variations is problematic, as the proposed indirect benefits to harbouring the parasite are surely the same in these populations, yet the prevalence is different. This fact illustrates the difficulties in interpreting variation in symbiont occurrence and is perhaps the main reason why there is little quantitative empirical evidence to date concerning the factors determining male killer prevalence.
Chapter 1: Introduction 37
Table 1.4. Prevalence of male killing bacteria in natural populations of their insect hosts. Prevalence is expressed as the percentage of females infected, n>30 in all cases.
Host Prevalence (%) Reference
Adalia bipunctata Spiroplasma: 0-22
Wolbachia : 0- 5
Rickettsia : 5 -1
Hurst et a l, 1999c Hurst et a l, 1999a
Hurst et a l, 1999c; Hurst et a l, 1993
Harmonia axyridis 0-49 Majerus et a l, 1998
Danaus chrysippus 40 Jiggins et a l, 2000b
Drosophila willistoni 0-3 Williamson & Poulson, 19^79
Acraea encedon 61-95 Jiggins cr a/., 1998= - -
Acraea encedana 95 Jiggins et a l , 2000a
Coleomegilla maculata 23 Hurst et a l, 1996b
Brachys tessellatus 50 Lawson et a l , 2001
Nasonia vitripennis 4 Skinner, 1985
Drosophila bifasciata 0-7 Ikeda, 1970
Gastrolina depresa 0-50 Chang era/., 1991