Biology subjects

Loytynoja, A.

Publications and source records attributed to Loytynoja, A..

11 recordsLinked to original sources

bioRxiv2026

A chromosome-level reference genome assembly reveals extreme loss of Y-chromosomal diversity in the Saimaa ringed seal (Pusa saimensis)

Saimaa ringed seals (Pusa saimensis) are a highly endangered freshwater pinniped species. Due to their unique evolutionary history and intense persecution over the last century, their population size and genetic diversity are low. Although mitochondrial and genome-wide variation have been extensively investigated, the paternal evolutionary history and genetic diversity of the Y chromosome remain poorly understood due to the lack of a suitable reference sequence. In this study, we take advantage of a new chromosome-level reference genome containing a curated Y chromosome and whole-genome resequencing data from 80 individuals, and characterize Y-chromosomal diversity, compare paternal and maternal lineages, and validate genomic approaches for sex determination. Read-depth analyses confirmed the accuracy of the assembled sex chromosomes, with male individuals exhibiting the expected two-fold increase in coverage across the pseudoautosomal region of chromosome X and exclusive coverage of the Y chromosome. These coverage patterns enabled reliable molecular sex determination. Phylogenetic analyses showed that both Y chromosome and mitochondrial haplotypes of the Saimaa ringed seal form distinct monophyletic clades relative to Baltic, Lake Ladoga and Arctic ringed seals, consistent with the long evolutionary isolation of the species. Similarly, the extent of genetic diversity in the Y chromosome and the mitochondrion aligned with with this long isolation history, since relative to the diversity observed across non-Saimaa ringed seals, the contemporary Saimaa population was estimated to retain only 5% of mitochondrial diversity and approximately 1% of Y-chromosomal diversity. All together, we show that chromosome-scale reference genome assemblies that include complete sex chromosomes provide a valuable resource for conservation genomics, studies of sex-specific population history, and comparative investigations of Y chromosome evolution across pinnipeds and other mammal species.

genomics↗

bioRxiv2025

Deep origins and distinct adaptations indicated for a glacial relict seal

Isolated populations of postglacial relicts are known from many regions and are typically found on mountains for terrestrial species and in lakes for aquatic species. Among the few aquatic mammalian relicts, the Saimaa ringed seal (Pusa hispida saimensis) has been landlocked in Lake Saimaa, Finland, for the last 10,000 years. Saimaa ringed seals show genetic, behavioral, and morphological differences to the other ringed seal subspecies, but the extent these differences stem from the end of the last glacial period remains unclear. Here, we demonstrate with comprehensive sampling and state-of-the-art genomic methods that the Saimaa ringed seals are much older than the lake they inhabit, having formed a separate evolutionary branch for at least 60,000 years. This deep evolutionary origin of the Saimaa ringed seals is further underscored by our ecomorphological analyses revealing adaptively distinct features in their dentition and tongue. Overall, glacial relicts, many threatened by extinction, may harbor a richer selection of evolutionary history than might be expected from their recent isolation history alone.

zoology↗

bioRxiv2023

Secondary contact, introgressive hybridization and genome stabilization in sticklebacks

Advances in genomic studies have revealed that hybridization in nature is pervasive and raised questions about the dynamics of different genetic and evolutionary factors following the initial hybridization event. While recent research has proposed that the genomic outcomes of hybridization might be predictable to some extent, many uncertainties remain. With comprehensive whole-genome sequence data, we investigated the genetic introgression between two divergent lineages of nine-spined sticklebacks (Pungitius pungitius) in the Baltic Sea. We found that the intensity and direction of selection on the introgressed variation varied across different genomic elements: while functionally important regions had experienced reduced rates of introgression, promoter regions showed enrichment. Despite the general trend of negative selection, we identified specific genomic regions that were enriched for introgressed variants and within these regions, we detected footprints of selection, indicating adaptive introgression. We found the selection against the functional changes to be strongest in the vicinity of the secondary contact zone and weaken as a function of distance from the initial contact. Altogether, the results suggest that the stabilization of introgressed variation in the genomes is a complex, multi-stage process involving both negative and positive selection. In spite of the predominance of negative selection against introgressed variants, we also found evidence for adaptive introgression variants likely associated with adaptation to Baltic Sea environmental conditions.

genomics↗

bioRxiv2023

Estimating recent and historical effective population size of marine and freshwater sticklebacks

Effective population size (Ne) is a quantity of central importance in evolutionary biology and population genetics, but often notoriously challenging to estimate. Analyses of Ne are further complicated by the many interpretations of the concept and the alternative approaches to quantify Ne utilising different properties of the data. However, the alternative methods are also informative over different time scales, suggesting that a combination of approaches should allow piecing together the entire continuum of Ne, spanning from the recent to more distant past. To test this in practice, we inferred the Ne continuum for 45 populations of nine-spined sticklebacks (Pungitius pungitius) using whole-genome data with both LD- and coalescent-based methods. Our results show that marine populations exhibit the highest Ne values in contemporary, recent, and historical times, followed by coastal and freshwater populations. They also demonstrate the impact of both recent and historical gene flow on Ne estimates and show that simple summary statistics are informative in comprehending the events in the very recent past and aid in more accurate estimation of NeC, the contemporary Ne, as well as in reconstruction and interpretation of recent demographic histories. Although our sample size for large populations is limited, we found that GONE can provide reasonable Ne estimates. However, due to challenges in detecting subtle genetic drift in large populations, these estimates may represent the lower bound of Ne. Finally, we show that combining GONE and CurrentNe2, both sensitive to population structure, with MSMC2 provides a meaningful interpretation of Ne dynamics over time.

genomics↗

bioRxiv2023

Generation of de novo miRNAs from template switchingduring DNA replication

The mechanisms generating novel genes and genetic information are poorly known, even for microRNA (miRNA) genes with an extremely constrained design. All miRNA primary transcripts need to fold into a stem-loop structure to yield short gene products ([~]22 nt) that bind and reppress their mRNA targets. While a substantial number of miRNA genes are ancient and highly conserved, short secondary structures coding for entirely novel miRNA genes have been shown to emerge in a lineage-specific manner. Template switching is a DNA-replication-related mutation mechanism that can introduce complex changes and generate perfect base pairing for entire hairpin structures in a single event. Here we show that the template-switching mutations (TSMs) have participated in the emergence of over 6,000 suitable hairpin structures in the primate lineage to yield at least 18 new human miRNA genes, that is 26% of the miRNAs inferred to have arisen since the origin of primates. While the mechanism appears random, the TSM-generated miRNAs are enriched in introns where they can be expressed with their host genes. The high frequency of TSM events provides raw material for evolution. Being orders of magnitude faster than other mechanisms proposed for de novo creation of genes, TSM-generated miRNAs enable near-instant rewiring of genetic information and rapid adaptation to changing environments.

genetics↗

bioRxiv2023

Determinants of genetic diversity in sticklebacks

Understanding what determines species and population differences in levels of genetic diversity has important implications for our understanding of evolution, as well as for the conservation and management of wild populations. Previous comparative studies have emphasized the roles of linked selection, life-history trait variation and genomic properties, rather than pure demography, as important determinants of genetic diversity. However, these findings are based on coarse estimates across a range of highly diverged taxa, and it is unclear how well they represent the processes within individual species. We assessed genome-wide genetic diversity ({pi}) in 45 nine-spined stickleback (Pungitius pungitius) populations and found that{pi} varied 15-fold among populations ({pi}min{approx}0.00015,{pi} max{approx}0.0023) whereas estimates of recent effective population sizes varied 122-fold. Analysis of inbreeding coefficients (FROH) estimated from runs of homozygosity revealed strong negative association between{pi} and FROH. Genetic diversity was also negatively correlated with mean body size and longevity, but these associations were not statistically significant after controlling for demographic effects (FROH). The results give strong support for the view that populations demographic features, rather than life history differences, are the chief determinants of genetic diversity in the wild.

evolutionary biology↗

bioRxiv2021

Detailed phylogeographic analysis of sticklebacks exposes challenges in genetic studies of admixed populations

Introgressive hybridization is an important process in evolution but challenging to identify, undermining the efforts to understand its role and significance. On the other hand, many analytical methods assume direct descent from a single common ancestor, and admixture among populations can violate their assumptions and lead to seriously biased results. A detailed analysis of 888 whole genome sequences of nine-spined sticklebacks (Pungitius pungitius) revealed a complex pattern of population ancestry involving multiple waves of gene flow and introgression across northern Europe. The two recognized lineages were found to have drastically different histories and their secondary contact zone was wider than anticipated, displaying a smooth gradient of foreign ancestry with some curious deviations from the expected pattern. Interestingly, the freshwater isolates provided peeks into the past and helped to understand the intermediate states of evolutionary processes. Our analyses and findings paint a detailed picture of the complex colonization history of northern Europe and provide back-drop against which introgression and its role in evolution can be investigated. However, they also expose the challenges in analyses of admixed populations and demonstrate how hidden admixture and colonization history misleads the estimation of admixture proportions and population split times.

genomics↗

bioRxiv2021

Template switching in DNA replication can create and maintain RNA hairpins

The evolutionary origin of ribonucleic acid (RNA) stem structures and the preservation of their base-pairing under a spontaneous and random mutation process have puzzled theoretical evolutionary biologists. DNA replication-related template switching is a mutation mechanism that creates reverse-complement copies of sequence regions within a genome by replicating briefly either along the complementary or nascent DNA strand. Depending on the relative positions and context of the four switch points, this process may produce a reverse-complement repeat capable of forming the stem of a perfect DNA hairpin, or fix the base-pairing of an existing stem. Template switching is typically thought to trigger large structural changes and its possible role in the origin and evolution of RNA genes has not been studied. Here we show that the reconstructed ancestral histories of RNA genes contain mutation patterns consistent with the DNA replication-related template switching. In addition to multi-base compensatory mutations, the mechanism can explain complex sequence changes, though mutations breaking the structure rarely get fixed in evolution. Our results suggest a solution for the longstanding dilemma of RNA gene evolution and demonstrate how template switching can both create perfect stems with a single mutation event and help maintaining the stem structure over time. Interestingly, template switching also provides an elegant explanation for the asymmetric base-pair frequencies within RNA stems.

evolutionary biology↗

bioRxiv2020

Convergent genetic architecture underlies parallel pelvic reduction in genetically highly structured stickleback species

Repeated and independent adaptation to specific environmental conditions from standing genetic variation is common. However, if genetic variation is limited, the evolution of similar locally adapted traits may be restricted to genetically different and potentially less optimal solutions or prevented from happening altogether. Using a quantitative trait locus (QTL) mapping approach, we identified the genomic regions responsible for the repeated pelvic reduction (PR) in three crosses between nine-spined stickleback populations expressing full and reduced pelvic structures. In one cross, PR mapped to linkage group 7 (LG7) containing the gene Pitx1, known to control pelvic reduction also in the three-spined stickleback. In the two other crosses, PR was polygenic and attributed to ten novel QTL, of which 90% were unique to specific crosses. When screening the genomes from 27 different populations for deletions in the Pitx1 regulatory element, these were only found in the population in which PR mapped to LG7, even though the morphological data indicated large effect QTL for PR in several other populations as well. Consistent with the available theory and simulations parameterised on empirical data, we hypothesise that the observed variability in genetic architecture of PR is due to heterogeneity in the spatial distribution of standing genetic variation caused by >2x stronger population structuring among freshwater populations and >10x stronger genetic isolation by distance in the sea in nine-spined sticklebacks as compared to three-spined sticklebacks.

evolutionary biology↗

bioRxiv2019

Genome sequencing of the nine-spined stickleback (Pungitius pungitius) provides insights into chromosome evolution.

The Gasterostidae fish family hosts several species that are important models for eco-evolutionary, genetic and genomic research. In particular, a wealth of genetic and genomic data have been generated for the three-spined stickleback (Gasterosteus aculeatus), the ecologys supermodel, while the genomic resources for the nine-spined stickleback (Pungitius pungitius) have remained relatively scarce. Here, we report a high-quality chromosome-level genome assembly of P. pungitius consisting of 5,303 contigs (N50 = 1.2 Mbp) with a total size of 521 Mbp. These contigs were mapped to 21 linkage groups using a high-density linkage map, yielding a final assembly with 98.5% BUSCO completeness. A total of 25,062 protein-coding genes were annotated, and ca. 23% of the assembly was found to consist of repetitive elements. A comprehensive analysis of repetitive elements uncovered centromeric-specific tandem repeats and provided insights into the evolution of retrotransposons. A multigene phylogenetic analysis inferred a divergence time of about 26 million years (MYA) between nine- and three-spined sticklebacks, which is far older than the commonly assumed estimate of 13 MYA. Compared to the three-spined stickleback, we identified an additional duplication of several genes in the hemoglobin cluster. Sequencing data from populations adapted to different environments indicated potential copy number variations in hemoglobin genes. Furthermore, genome-wide synteny comparisons between three- and nine-spined sticklebacks identified chromosomal rearrangements underlying the karyotypic differences between the two species. The high-quality chromosome-scale assembly of the nine-spined stickleback genome obtained with long-read sequencing technology provides a crucial resource for comparative and population genomic investigations of stickleback fishes and teleosts.

genomics↗

bioRxiv2018

Bracketing phenotypic limits of mammalian hybridization

An increasing number of mammalian species have been shown to have a history of hybridization and introgression based on genetic analyses. Only relatively few fossils, however, preserve genetic material and morphology must be used to identify the species and determine whether morphologically intermediate fossils could represent hybrids. Because dental and cranial fossils are typically the key body parts studied in mammalian paleontology, here we bracket the potential for phenotypically extreme hybridizations by examining uniquely preserved cranio-dental material of a captive hybrid between gray and ringed seals. We analyzed how distinct these species are genetically and morphologically, how easy it is to identify the hybrids using morphology, and whether comparable hybridizations happen in the wild. We show that the genetic distance between these species is more than twice the modern human-Neanderthal distance, but still within that of morphologically similar species-pairs known to hybridize. In contrast, morphological and developmental analyses show gray and ringed seals to be highly disparate, and that the hybrid is a predictable intermediate. Genetic analyses of the parent populations reveal introgression in the wild, suggesting that gray-ringed seal hybridization is not limited to captivity. Taken together, gray and ringed seals appear to be in an adaptive radiation phase of evolution, showing large morphological differences relative to their comparatively modest genetic distance. Because morphological similarity does not always correlate with genetic distance in nature, we postulate that there is considerable potential for mammalian hybridization between phenotypically disparate taxa.

evolutionary biology↗