Investigating sex differences in genetic relatedness in great-tailed grackles in Tempe, Arizona to infer potential sex biases in dispersal
Sevchik A (Arizona State University), Logan CJ (Max Planck Institute for Evolutionary Anthropology, MPI EVA), Folsom M (MPI EVA), Bergeron L (University of California Santa Barbara), Blackwell A (Washington State University), Rowney C (MPI EVA), Lukas D (MPI EVA, dieter.lukas@eva.mpg.de)
2019-12-18
#Make code wrap text so it doesn't go off the page when Knitting to PDF
library(knitr)
opts_chunk$set(tidy.opts=list(width.cutoff=60),tidy=TRUE)
Cite as: Sevchik A, Logan CJ, Bergeron L, Blackwell A, Rowney C, Lukas D. 2019. Investigating sex differences in genetic relatedness in great-tailed grackles in Tempe, Arizona to infer potential sex biases in dispersal. (http://corinalogan.com/Preregistrations/gdispersal.html) In principle acceptance by PCI Ecology of the version on 29 Nov 2019 https://github.com/corinalogan/grackles/blob/master/Files/Preregistrations/gdispersal.Rmd.
This preregistration has been pre-study peer reviewed and received an In Principle Recommendation by:
Sophie Beltran-Bech (2019 In Principle Acceptance) Investigate fine scale sex dispersal with spatial and genetic analyses. Peer Community in Ecology, 100036. 10.24072/pci.ecology.100036
- Reviewers: Sylvine Durand and one anonymous reviewer
ABSTRACT
In most bird species, females disperse prior to their first breeding attempt, while males remain close to the place they were hatched for their entire lives (Greenwood and Harvey (1982)). Explanations for such female bias in natal dispersal have focused on the potential benefits that males derive from knowing the local environment to establish territories, while females search for suitable mates (Greenwood (1980)), however the exact factors shaping dispersal decisions appear more complex (Mabry et al. (2013), Végvári et al. (2018)). Here, we investigate whether females are the dispersing sex in great-tailed grackles, which have a mating system where the males hold territories and the females choose which territory to place their nest in (Johnson et al. (2000)). We will use genetic approaches to identify sex biases in the propensity to disperse. We will first determine whether, for individuals caught at a single site in Arizona, the average relatedness among all female dyads is lower than that among all male dyads. If supported, this would suggest that females are less likely to be found close to genetic relatives, which indicates that females disperse away from relatives. Second, we will assess whether in males close relatives are most likely to be found within very short distances of each other; whereas, in females, relatives live both near and far from each other. Results will inform our long-term study on the relationship between behavioral flexibility and rapid geographic range expansion by elucidating which individuals are likely to experience similar conditions across their lives, and which are likely to face new conditions when they become breeders.
A. STATE OF THE DATA
The first version of this preregistration was written (June 2019) and submitted to Peer Community In Ecology (July 2019) after blood was collected and before processing the DNA to obtain the genetic data. The revised version of this preregistration following peer review at Peer Community in Ecology (October 2019) was written after obtaining the genotypes for the individuals in the sample and resubmitted in November 2019.
B. HYPOTHESIS
Hypothesis There are sex differences in the natal disperal rate and distance among individuals in great-tailed grackles (Quiscalus mexicanus) with males remaining close to where they hatched and females moving away from where they hatched. Males are expected to remain close to the area where they hatched, therefore a large number of the males on the Arizona State University (ASU) campus are expected to have hatched within the area of the study site and stay close to their relatives. In contrast, females are expected to move before their first breeding attempt (Greenwood (1980)), therefore females on campus are likely to come from areas outside of campus in the surrounding area, having moved away from relatives.
Alternative 1 Males disperse away from where they hatched, while females remain where they hatched.
Alternative 2 Individuals of both sexes remain close to where they hatched.
Alternative 3 Individuals of both sexes disperse away from where they hatched.
We predict that the movement of individuals will influence the spatial distribution of genetic relatives. Individuals of the sex who remain close to where they hatched are expected to be close to relatives while individuals of the sex who disperse are expected to not be close to relatives (see Fig 1 for a visualization). We also expect that the further the distance an individual moves, the less likely they are to be even distantly related to another individual within the study area. We will perform three analyses to investigate the spatial distribution of genetic relatives: the first two aim to detect whether there are sex biases in levels of average genetic relatedness among indivduals found within a certain distance of each other (analysis i: average levels of relatedness among individuals in our sample; analysis ii: geographic distances between individuals assumed to be genetic relatives) and the third aims to describe the genetic structure separately for each sex and whether relatives are predominantly found within certain distances from each other or whether relatives are not structured in geographic space (analysis iii: spatial autocorrelation).
Predictions for the hypothesis: dispersal males < females - Analysis i (average relatedness): Both the mean level of and the variance in average genetic relatedness will be higher among males compared to females in our sample. - Analysis ii (distance between relatives): Both the mean and variance of the geographic distances between pairs of individuals inferred to be genetic relatives (individuals estimated to be related at levels of cousins or closer, r>0.125) will be shorter among males compared to females. - Analysis iii (spatial autocorrelation): There will be a spatial autocorrelation signal indicating a negative relationship between genetic relatedness and geographic distance for males. There will be no spatial autocorrelation signal indicating the absence of a relationship between genetic relatedness and geographic distance for females.
Predictions for alternative 1: dispersal males > females - Analysis i (average relatedness): Both the mean level of and the variance in average genetic relatedness will be lower among males compared to females in our sample. - Analysis ii (distance between relatives): Both the mean and variance of the geographic distances between pairs of individuals inferred to be genetic relatives (individuals estimated to be related at levels of cousins or closer, r>0.125) will be higher among males compared to females. - Analysis iii (spatial autocorrelation): There will be no spatial autocorrelation signal indicating the absence of a relationship between genetic relatedness and geographic distance for males. There will be a spatial autocorrelation signal indicating a negative relationship between genetic relatedness and geographic distance for females.
Predictions for alternative 2: neither males nor females disperse - Analysis i (average relatedness): Both the mean level of and the variance in average genetic relatedness will be similar among males compared to females in our sample. - Analysis ii (distance between relatives): Both the mean and variance of the geographic distances between pairs of individuals inferred to be genetic relatives (individuals estimated to be related at levels of cousins or closer, r>0.125) will be similar among males compared to females. - Analysis iii (spatial autocorrelation): There will be a spatial autocorrelation signal indicating a negative relationship between genetic relatedness and geographic distance for males. There will be a spatial autocorrelation signal indicating a negative relationship between genetic relatedness and geographic distance for females.
Predictions for alternative 3: both males and females disperse - Analysis i (average relatedness): Both the mean level of and the variance in average genetic relatedness among males will be similar to that among females. - Analysis ii (distance between relatives): Both the mean and variance of the geographic distances between pairs of males inferred to be genetic relatives (individuals estimated to be related at levels of cousins or closer, r>0.125) will be similar to distances among female genetic relatives. - Analysis iii (spatial autocorrelation): There will be no spatial autocorrelation signal indicating the absence of a relationship between genetic relatedness and geographic distance for males. There will be no spatial autocorrelation signal indicating the absence of a relationship between genetic relatedness and geographic distance for females.
Figure 1. Visual representation of the hypothesis that males will have higher levels of genetic relatedness than females because females likely have larger dispersal distances.
C. METHODS
Planned Sample
DNA from 57 great-tailed grackles was obtained from wild adults (n=40 adult females, n=17 adult males, juvenile samples were excluded because they had not yet dispersed) caught in Tempe, Arizona, USA (see Fig 2 for a map). These individuals were either immediately released, or temporarily brought into aviaries for behavioral testing and then released back to the wild.
Figure 2. Map displaying the sampling locations of grackles on the ASU campus and the number of individuals trapped at each location as part of this project.
The larger number of females than males in our sample appears to reflect the adult sex ratio at this study site. To estimate the sex ratio at the field site, we counted the number of females and males that were trapped in mist nets since the beginning of our study (September 2017 - October 2019). This trapping method likely does not elicit a sex bias in terms of which sex is caught because the nets are invisible. Therefore, if one sex is more neophobic than the other, both sexes are likely to be trapped using this method. A total of 26 females and 11 males were trapped using mist nets (a ratio of 2.36 females per 1 male), which is very similar to the sex ratio in our sample consisting of 40 females and 17 males (2.35 females per 1 male).
Females were caught at all but one site, such that comparisons are possible of the genetic relatedness of pairs of females trapped at various distances from each other. Males were not caught at all trap sites, but there are several sites at which multiple males were caught and sufficient sites for comparisons of males that were caught close to each other, at intermediate, and at long distances (Figure 3).