Causes of spatial variation in parasite and pathogen pressure in insects

The reproduction of the two-spot ladybird, Adalia bipunctata, is inhibited by a sexually transmitted ectoparasitic mite Coccipolipus hippodamiae that sterilizes female hosts, and a range of heritable microbes that kill male hosts during embryogenesis for instance Spiroplasma. Historical sampling indicated these two parasites were present commonly in A. bipunctata in the south of Sweden but were absent or rare in northern populations. In this thesis, I first established that the distribution of the mite was still stable over time, with a boundary to mite presence at 61˚N, as found 10 years previously. The incidence of male-killing Spiroplasma, in contrast, showed a small northward shift in its presence. I then examined the causes of mite presence/absence, and concluded ladybirds from northern populations were competent to carry and transmit mite infection, but that host phenology prevented its persistence in natural populations. I then explored the hypothesis that mite presence might select for increased reluctance of females to mate by comparing mating behaviour between mite present/mite absent populations. I observed that whilst rejection behaviour was protective against mite transmission, it was not more commonly observed in ladybirds derived from mite present populations. Finally, I examined whether the presence of male-killing Spiroplasma might affect mite epidemiology, and whether the mite itself might impact on the dynamics of the male-killing Spiroplasma. I observed that Spiroplasma did not affect individual competence to acquire and transmit mite infection, and did not protect its host against mite induced sterility. However, Spiroplasma was observed to mildly prolong host longevity, which may enhance the capacity of the mite to pass from overwintered to new generation cohorts of its host. Further, population sex ratio biases induced by Spiroplasma were predicted to influence mite epidemiology, through altering mating rate and per contact mite transmission probability. I therefore conclude first that mite incidence can be explained through host phenological variation, and that sex ratio distorting symbionts may impact on the dynamics of the mite infection. Future studies should explore the causes of high prevalence Spiroplasma infection in A. bipunctata in southern Sweden, and the features that prevent the establishment of infection in the north of the country

Variation in mycorrhizal growth response among a spring wheat mapping population shows potential to breed for symbiotic benefit.

Funder: N8 Agrifood SchemeAll cereal crops engage in arbuscular mycorrhizal symbioses which can have profound, but sometimes deleterious, effects on plant nutrient acquisition and growth. The mechanisms underlying variable mycorrhizal responsiveness in cereals are not well characterised or understood. Adapting crops to realise mycorrhizal benefits could reduce fertiliser requirements and improve crop nutrition where fertiliser is unavailable. We conducted a phenotype screen in wheat (Triticum aestivum L.), using 99 lines of an Avalon × Cadenza doubled-haploid mapping population. Plants were grown with or without a mixed inoculum containing 5 species of arbuscular mycorrhizal fungi. Plant growth, nutrition and mycorrhizal colonisation were quantified. Plant growth response to inoculation was remarkably varied among lines, ranging from more than 30% decrease to 80% increase in shoot biomass. Mycorrhizal plants did not suffer decreasing shoot phosphorus concentration with increasing biomass as observed in their non-mycorrhizal counterparts. The extent to which mycorrhizal inoculation was beneficial for individual lines was negatively correlated with shoot biomass in the non-mycorrhizal state but was not correlated with the extent of mycorrhizal colonisation of roots. Highly variable mycorrhizal responsiveness among closely related wheat lines and the identification of several QTL for these traits suggests the potential to breed for improved crop-mycorrhizal symbiosis

No evidence that presence of sexually transmitted infection selects for reduced mating rate in the two spot ladybird, Adalia bipunctata

Sexually transmitted infections (STIs) are common in animals and plants, and frequently impair individual fertility. Theory predicts that natural selection will favour behaviours that reduce the chance of acquiring a STI. We investigated whether an STI, Coccipolipus hippodamiae has selected for increased rejection of mating by female Adalia bipunctata as a mechanism to avoid exposure. We first demonstrated that rejection of mating by females did indeed reduce the chance of acquiring the mite. We then examined whether rejection rate and mating rate differed between ladybirds from mite-present and mite-absent populations when tested in a common environment. No differences in rejection intensity or remating propensity were observed between the two populations. We therefore conclude there is no evidence that STIs have driven the evolution of female mating behaviour in this species

Data from: The role of host phenology in determining the incidence of an insect sexually transmitted infection

Changes in the timing of life history events within the year alter the degree to which the activity patterns of different species coincide, making the dynamics of interspecific interactions sensitive to the phenology of the interacting parties. For parasites, the availability of suitable hosts to infect represents a crucial determinant of dynamics, and changes in the host (and parasite) phenology may thus alter disease epidemiology and the conditions for disease maintenance. We tested the hypothesis that the incidence of a sexually transmitted mite infection, Coccipolipus hippodamiae, in Adalia bipunctata ladybird beetles in Sweden was determined by host phenology, namely presence/absence of sexual contact between cohorts of the host. We observed that the pattern of mite presence/absence across Swedish A. bipunctata populations was highly reproducible between years, implying a persistent biological/ecological basis underlying the incidence. Further, ladybirds from populations where the mite was absent were able to acquire mites during copulation, develop a mite infection, and transmit infection onward, indicating an ecological (rather than biological) driver of mite incidence. Observations of ladybird phenology in natural populations provided evidence of sexual contact between overwintered and new cohort adults in populations where the mite was present. In contrast, new cohort ladybirds in the two northern Swedish populations where the mite was not present had not had sexual contact with the overwintered generation, creating a ‘hard stop’ to mite transmission. We conclude that variation in host phenology may be an important driver of the incidence of sexually transmitted infections (STIs) by determining the presence/absence of sexual contact between generations. More generally, we hypothesize that sensitivity to variation in host phenology will be highest for parasites like STIs that infect one host species, one host life stage and are directly transmitted on contact between host individuals

The role of host phenology in determining the incidence of an insect sexually transmitted infection

the NERC. Grant Number: NE/G003246/1 Univ. of Liverpool studentship (DP

Pastok et al Stockholm phenology

Data supporting Figure 2 and Figure 3

Raw_epidemic_data

Data supporting figure 4 in the journal. Prevalence of C. hippodamiae on A. bipunctata beetles in a series of epidemics, partitioned by A. bipunctata sex