Novel insights into symbiont population structure: Globe-trotting avian feather mites contradict the specialist-generalist variation hypothesis.

Researchers often examine symbiont host specificity as a species-level pattern, but it can also be key to understanding processes occurring at the population level, which are not as well understood. The specialist-generalist variation hypothesis (SGVH) attempts to explain how host specificity influences population-level processes, stating that single-host symbionts (specialists) exhibit stronger population genetic structure than multi-host symbionts (generalists) because of fewer opportunities for dispersal and more restricted gene flow between populations. However, this hypothesis has not been tested in systems with highly mobile hosts, in which population connectivity may vary temporally and spatially. To address this gap, we tested the SGVH on proctophyllodid feather mites found on migratory warblers (family Parulidae) with contrasting host specificities, Amerodectes protonotaria (a host specialist of Protonotaria citrea) and A. ischyros (a host generalist of 17 parulid species). We used a pooled-sequencing approach and a novel workflow to analyse genetic variants obtained from whole genome data. Both mite species exhibited fairly weak population structure overall, and contrary to predictions of the SGVH, the generalist was more strongly structured than the specialist. These results may suggest that specialists disperse more freely among conspecifics, whereas generalists sort according to geography. Furthermore, our results may reflect an unexpected period for mite transmission - during the nonbreeding season of migratory hosts - as mite population structure more closely reflects the distributions of hosts during the nonbreeding season. Our findings alter our current understanding of feather mite biology and highlight the potential for studies to explore factors driving symbiont diversification at multiple evolutionary scales.

Draft genome sequencing data of a feather mite, Amerodectes protonotaria Hernandes 2018 (Acariformes: Proctophyllodidae).

Feather mites are ubiquitous, permanent, obligate ectosymbionts of avian hosts and are a valuable natural system for studying host-symbiont evolutionary and ecological dynamics at multiple levels of biological organization. However, a lack of a sequenced genome impedes molecular studies using this system. Therefore, we present the first draft genome of a symbiotic feather mite, Amerodectes protonotaria Hernandes 2018. The genome sequence data presented here were derived from an individual female mite that was collected in the field from Protonotaria citrea, its only known host species. Short read sequence data were obtained using an Illumina NovaSeq 6000 platform. From these data, we assembled a 59,665,063 bp draft genome consisting of 2,399 contigs. Raw short reads and the assembled genome sequence are available at the National Center for Biotechnology Information (NCBI)'s Sequence Read Archive (SRA) under BioProject PRJNA884722. The data presented here are beneficial for future research on the biology and evolution of closely related mites and the genomics of host-symbiont interactions.

Population Genomics of Pooled Samples: Unveiling Symbiont Infrapopulation Diversity and Host-Symbiont Coevolution.

Microscopic symbionts represent crucial links in biological communities. However, they present technical challenges in high-throughput sequencing (HTS) studies due to their small size and minimal high-quality DNA yields, hindering our understanding of host-symbiont coevolution at microevolutionary and macroevolutionary scales. One approach to overcome those barriers is to pool multiple individuals from the same infrapopulation (i.e., individual host) and sequence them together (Pool-Seq), but individual-level information is then compromised. To simultaneously address both issues (i.e., minimal DNA yields and loss of individual-level information), we implemented a strategic Pool-Seq approach to assess variation in sequencing performance and categorize genetic diversity (single nucleotide polymorphisms (SNPs)) at both the individual-level and infrapopulation-level for microscopic feather mites. To do so, we collected feathers harboring mites (Proctophyllodidae: Amerodectes protonotaria) from four individual Prothonotary Warblers (Parulidae: Protonotaria citrea). From each of the four hosts (i.e., four mite infrapopulations), we conducted whole-genome sequencing on three extraction pools consisting of different numbers of mites (1 mite, 5 mites, and 20 mites). We found that samples containing pools of multiple mites had more sequencing reads map to the feather mite reference genome than did the samples containing only a single mite. Mite infrapopulations were primarily genetically structured by their associated individual hosts (not pool size) and the majority of SNPs were shared by all pools within an infrapopulation. Together, these results suggest that the patterns observed are driven by evolutionary processes occurring at the infrapopulation level and are not technical signals due to pool size. In total, despite the challenges presented by microscopic symbionts in HTS studies, this work highlights the value of both individual-level and infrapopulation-level sequencing toward our understanding of host-symbiont coevolution at multiple evolutionary scales.

Bergmann's rule is followed at multiple stages of postembryonic development in a long-distance migratory songbird.

Bergmann's rule is a well-established, ecogeographical principle that states that body size varies positively with latitude, reflecting the thermoregulatory benefits of larger bodies as temperatures decline. However, this principle does not seem to easily apply to migratory species that are able to avoid the extreme temperatures during winter at higher latitudes. Further, little is known about the ontogeny of this relationship across life stages or how it is influenced by ongoing global climate change. To address these knowledge gaps, we assessed the contemporary relationship between latitude and body size in a long-distance migratory species, the prothonotary warbler (Protonotaria citrea) across life stages (egg to adult) on their breeding grounds. We also measured historic eggs (1865-1961) to assess if the relationship between latitude and size during this life stage has changed over time. In accordance with Bergmann's rule, we found a positive relationship between latitude and body mass during all post-embryonic life stages, from early nestling stage through adulthood. We observed this same predicted pattern with historic eggs, but contemporary eggs exhibited the reverse (negative) relationship. We suggest that these results indicate a genetic component to this pattern and speculate that selection for larger body size in altricial nestlings as latitude increases may possibly drive the pattern in migratory species as even rare extreme cold weather events may cause mortality during early life stages. Furthermore, the opposite relationships observed in eggs, dependent on time period, may be related to the rapidly warming environments of higher latitudes that is associated with climate change. Although it is unclear what mechanism(s) would allow for this recent reversal in eggs (but still allow for its maintenance in later life stages). This evidence of a reversal suggests that anthropogenic climate change may be in the process of altering one of the longest-standing principles in ecology.