3.1 Vertical transmission during asexual reproduction
Each of the 336 asexual offspring from infected females from 19 D. magna
clones was infected (), confirming an earlier smaller experiment on vertical transmission of O. bayeri
into asexual host offspring (Vizoso et al. 2005
Figure 1 Percentage of D. magna offspring infected with the microsporidian parasite O. bayeri from infected mothers who reproduced by parthenogenesis (asexual), selfing (monoclonal populations) or mostly outcrossing (polyclonal populations). Note, the selfing (more ...)
3.2 Vertical transmission during sexual reproduction
The number of hatchlings recovered in the mesocosms was between 0 and 299. In one monoclonal mesocosm, no hatchling was found and in three further monoclonal mesocosms, only 6, 13 and 19 individuals were caught. In all other populations, a minimum of 45 females were analysed. We found more hatchlings in the outbreeding (polyclonal) than in the selfing (monoclonal) treatment (2819 overall versus 1786), but this difference was not significant (Wilcoxon rank sum test: p=0.22). The average parasite prevalence among the selfed hatchlings (monoclonal mesocosm) was 98.9% (in total, 5 uninfected out of 364 tested individuals), with only two populations having less than 100% prevalence (92.3 and 98%). In contrast, uninfected hatchlings were found in all 11, mostly outbreeding populations (146 uninfected among 1006 tested individuals; the larger number of animals results from two populations in which all hatchlings were tested). The average proportion of infected offspring in polyclonal mesocosms was significantly lower than in the monoclonal populations (85.2 versus 98.9%; Kruskal–Wallis test: Χ12=13.03, p<0.0003; ).
The fitness consequences of this reduced prevalence for the mostly outbred offspring are difficult to assess, therefore we will explore the epidemiological consequences below with the help of observational data and an epidemiological model.
3.3 Within-season infection dynamics
The five rock pool populations varied in their spring prevalence, ranging from 35 to 75%. We observed a reduction in prevalence in all five populations in the first one to two months, followed by an increase, eventually leading to 100% prevalence in all populations ().
Dynamics of prevalence of the microsporidium O. bayeri in five natural rock pool populations of D. magna over one summer season.
3.4 An epidemiological model
Below, we introduce an epidemiological model for the within-season dynamics of O. bayeri
, which allows us to ask how the parasite influences the success of competitors with different starting prevalences across the asexual growth season. First, we largely follow the model by Lipsitch et al. (1995)
to model the within-season dynamics of a parasite with perfect vertical and some horizontal transmission through free-living spores in an asexual host population. The infection dynamics are described by the following three equations:Xi
are the uninfected and infected hosts; and S
is the number of free parasite spores. The subscript i
differentiates between two host types (competitors) 1 and 2. Hosts are born with a rate b
if uninfected and bf
if infected, with f
ranging from 0 to 1, indicating the degree to which infected hosts have a reduced fecundity. Birth rate is density dependent with N
being the total host population size and K
the carrying capacity. Hosts die with rate μ
for reasons unrelated to parasitism and with rate α
as a consequence of being infected. Healthy hosts may become infected by contact with free spores, S
, following the mass-action assumption with a rate β
. The mass-action assumption has been shown to satisfy transmission by free spores in Daphnia
–parasite systems (Regoes et al. 2003
). Free spores are produced when infected hosts die. We scale S
to be equal to the amount of spores released from one dead infected host. Spores die with rate g
For one host type (i=1), the model tracks the course of parasite spread during one host season (a). At the beginning of a season, hosts are born with a certain likelihood of being infected. As population size in early spring is usually far below the carrying capacity K, the population grows exponentially. Initially, horizontal transmission is rare, as X and S are small. Therefore, the parasite prevalence declines due to the lower fecundity of infected hosts (a,b). With increasing X and higher S, more and more hosts become infected and prevalence eventually rises to 100% (b). Prevalence fails to reach 100% only when the parasites have very strong effects on host fecundity (f<0.6 in c).
Figure 3 Dynamics of host and parasite in the epidemiological model for one host type. (a) Dynamics of total, infected and healthy host population over one summer season. (b) Dynamics of prevalence for different starting prevalences (from 10 to 90%) in the early (more ...)
In the next step, we allow two host types to compete (i=1 and 2). The two host types differ only in their initial early spring prevalence. Starting with the same initial population size, the two types diverge quickly so that the type that was initially less infected reaches a larger population size than the type that was initially more infected ().
Figure 4 Population size of two competing host types with an initial difference in their prevalence of infection. The solid lines indicate the population with 85% prevalence at the start of the season. The stippled line shows a competing host type with 98% prevalence. (more ...)
In Daphnia populations with an obligate resting stage, as is the case for the rock pool metapopulation considered here, the number of resting stages (equal to ephippia) produced over the summer season provides a good measure of the success of a certain strategy, as it determines the likelihood of surviving the harsh winter and determines the relative abundance at the start of the next season. If we assume that the ratio of population sizes of the two host types towards mid- or end season is a good predictor of their contribution to the resting egg bank (ephippia are usually produced in mid summer and towards end of season), we can estimate the relative seasonal success of one competitor relative to the other. shows the numerical ratio of two competitors with equal population size at the start of the season depending on their starting prevalence. If both types have the same starting prevalence, no difference is visible. However, a competitor with a lower initial prevalence has advantages in every case; this advantage is highest when the other competitor has an initial prevalence near 100%. These results are largely independent of the relative starting frequencies of the two competitors (the outbred genotypes may be initially rare), as long as the total starting density is kept constant.
The advantage of host type 1 over type 2 in relation to their prevalence at the beginning of the season. Parameter settings as in figure 3, except for the initial values of X and Y of the two host types.