title: Unravelling Cryptic Radiation and High-Altitude Adaptation in a Migratory Bird Species Complex
Creators: name: Tang, Qindong
Publisher: name: Universität Bern
Year: 2025
Subjects: subject: 570 Life sciences; biology

description: Biodiversity is often accompanied with drastic phenotypic diversity. Morphological differentiation can play a crucial role in the process of speciation, such as during adaptive radiation that ecological and phenotypic diversity evolved within rapidly multiplying lineages (Schluter, 2000). However, there is a considerable proportion of biodiversity that is constituted of cryptic species which are defined as evolutionary lineages with restricted gene flow that do not form diagnostic morphological clusters (Struck et al., 2018). Our understanding of cryptic diversity and mechanisms underlying the evolution of new species without distinguished morphological differentiation remains limited. Other traits could also play an important role in speciation. By studying how speciation happened under morphological stasis and how cryptic diversity can be maintained afterwards could help us to understand other mechanisms that restrict or prevent gene flow with minimal morphological and ecological differentiation.
This thesis mainly focused on the cryptic diversification of two migratory bird species complexes, pale sand martin Riparia diluta and collared sand martin Riparia riparia, with the aim of understanding the processes and mechanisms that lead to morphologically cryptic divergence and maintain genetic partitioning. I have conducted comprehensive geographical sampling for the two taxa in East Asia including the Qinghai-Tibetan Plateau and areas surrounding the Taklamakan desert.
In Chapter 1, by using genome-wide, morphology and phenology data, I revealed cryptic radiation in the pale sand martin in Central and East Asia. My results indicated that allochrony caused by different migration/breeding time and migratory divide might play an important role to restrict gene flow and maintain evolutionary diversity in birds under morphological stasis. In Chapter 2, I conducted phylogenomic reconstruction and demographic inferences to study the biogeography of pale sand martin and collared sand martin. Contrasting diversification pattern were found between the two sister taxa: mitochondrial-nuclear discordance in Holarctic collared sand martin versus strong genomic divergence in the pale sand martin supported the existence of multiple morphologically cryptic evolutionary with comparably limited distribution range in Asia. In Chapter 3, I investigated potential genomic adaptation to high altitudes in pale sand martin. By comparing the genomic differentiation between different populations breeding on different altitudes with large elevation change, I have identified EPAS1 as potential gene together with HBAD facilitating migratory birds' adaptation to high altitudes. Furthermore, my results indicated EPAS1 haplotypes from lowland subspecies R. d. diluta were likely adaptively introgressed into populations R. d. tibetana in central Mongolia that have expanded their range this area from the Qinghai-Tibetan plateau.
In this thesis, analyses of genome-wide data revealed cryptic radiation within a migratory bird species complex and revealed their genomic adaptation for high altitudes. My work indicated potential mechanism to restrict gene flow under cryptic diversification that in bird. We hypothesize that pronounced dispersal propensity in the strongly migratory nominate form of collared sand martin has hindered lineage divergence across its vast distribution range, while differential seasonal migratory behaviour contributes to maintain evolutionary diversity in the pale sand martin species complex. Furthermore my work indicated adaptive introgression can facilitate migratory bird to colonize new environment.