Biology subjects

Lifjeld, J. T.

Publications and source records attributed to Lifjeld, J. T..

5 recordsLinked to original sources

bioRxiv2026

Experimental evidence for antagonistic effects of sperm competition in two sea urchin species

Sperm competition can occur when sperm from two or more males compete for fertilization of a batch of eggs. While it has been extensively studied in internal fertilizers, it remains poorly under-stood in external fertilizers. To investigate how sperm competition affects fertilization under sperm limitation, we conducted controlled experiments on two sea urchin species: Paracentrotus lividus and Dendraster excentricus. Eggs from a single female were exposed to sperm from three individ-ual males separately (non-competitive treatment) or to a pooled sample from three males (competi-tive treatment). We measured fertilization success as proportion of eggs fertilized after two hours and assessed sperm motility as percentage of motile sperm and curvilinear velocity. For P. lividus , we estimated the sperm concentrations where fertilization success was 50% of maximum observed fertilization success (F50), and the fertilization efficiency of the sperm (Fe), i.e. the likelihood that a sperm - egg encounter results in a fertilizing contact, using the fertilization kinetics model of Styan (1998). For D. excentricus, fertilization success was measured at a sperm concentration expected to produce 50% fertilization success. Sperm competition reduced fertilization success and increased F50 compared to non-competitive treatments, suggesting antagonistic effects that may compromise fertilization under sperm-limited conditions in external fertilizers.

evolutionary biology↗

bioRxiv2026

A haplotype-resolved bluethroat (Luscinia s. svecica) genome assembly uncovers the complex MHC region

We describe a chromosome-level, haplotype-resolved genome assembly from a female bluethroat (Luscinia s. svecica). The assembly comprises two pseudo-haplotypes of 1461 Mb and 1171 Mb, with 77.4% and 88.4% scaffolded into 40 autosomal chromosomes and the W and Z sex chromosomes (haplotype one). Assembly completeness is high (BUSCO 99.2% and 94.9%), with 22,462 and 18,769 annotated protein-coding genes for haplotypes one and two, respectively. The use of Oxford Nanopore Technologies sequencing enables resolution of genomic regions that are often fragmented in genome assemblies, including the hypervariable Major Histocompatibility Complex (MHC). We find that MHC loci include both the canonical organization of tandemly duplicated MHCII{beta} genes with a single MHCIIA, and a distinct arrangement in which MHCI and MHCII{beta} loci are interspersed in intermixed arrays, and that substantial structural differences between haplotypes are directly resolved in the assembly.

genomics↗

bioRxiv2024

Assessing reliability and accuracy of qPCR, dPCR and ddPCR for estimating mtDNA copy number in songbird blood and sperm cells

Mitochondrial DNA copy number varies across species, individuals and cell types. Two avian cell types carrying a relatively low number of mitochondria are the red blood cells and spermatozoa. While previous studies investigating variation of mitochondrial abundance in animal sperm have generally used quantitative PCR (qPCR), this method shows potential limitations when quantifying low abundant targets. To mitigate such issues, we investigated and compared the reliability and accuracy of qPCR, digital PCR (dPCR) and droplet digital PCR (ddPCR) to quantify high and low concentration DNA. Using synthetic DNA, we found that both dPCR and ddPCR displayed lower Limit of Detection and Limit of Quantification than qPCR. Using DNA extracted from blood and sperm cells of Eurasian Siskin, we found that qPCR, dPCR and ddPCR reliably quantified mitochondrial DNA in sperm samples, but showed significant differences when analyzing typically lower levels of mtDNA in blood. We found that ddPCR consistently showed lower variation among replicates. These analyses provide critical insights and recommendations for future studies aiming to quantify target mtDNA. Our study indicates that dPCR and ddPCR are the preferred methods when working with samples with low abundance of mtDNA.

molecular biology↗

bioRxiv2024

Independent evolution of atypical sperm morphology in a passerine bird, the red-browed finch (Neochmia temporalis temporalis)

Spermatozoa exhibit striking morphological variation across the animal kingdom. In passerine birds, sperm exhibit considerable variation in size, yet the basic sperm phenotype is highly conserved; sperm are filiform, the head is corkscrew-shaped, and the midpiece is elongated and twisted around the flagellum. A significant departure from this typical sperm morphology has been reported in the sister species, the Eurasian bullfinch (Pyrrhula pyrrhula) and Azores bullfinch (P. murina). Here, we report a second evolutionary shift in passerine sperm phenotype in the nominate subspecies of the red-browed finch (Neochmia temporalis temporalis); sperm are non-filiform, with an ellipsoid head, and an extremely short midpiece restricted to the nuclear-axoneme junction. Additionally, we show that the sperm phenotype of the red-browed finch is similar to the putatively neotenous sperm described in the two bullfinch species. Using whole-genome data, we found no evidence that the unusual sperm phenotype of the red-browed finch is associated with reduced genetic variation or a population bottleneck. In contrast, we find some support for the hypothesis that relaxed post-copulatory sexual selection may, at least in part, explain the unusual sperm of the red-browed finch. We also discuss the possible roles of mutation, genetic drift, and genetic hitchhiking, in the evolutionary origins and maintenance of neotenous sperm phenotypes. Finally, we suggest that these dramatic evolutionary shifts in sperm phenotype warrant further investigation and highlight the need for a greater understanding of the developmental and genomic basis of sperm phenotype.

evolutionary biology↗

bioRxiv2022

Selection on sperm size in response to promiscuity and variation in female sperm storage organs

Sperm cells are exceptionally morphologically diverse across taxa. However, morphology can be quite uniform within species, particularly for species where females copulate with many males per reproductive bout. Strong sexual selection in these promiscuous species is widely hypothesized to reduce intraspecific sperm variation. Conversely, we hypothesize that intraspecific sperm size variation may be maintained by high among-female variation in the size of sperm storage organs, assuming that paternity success improves when sperm are compatible in size with the sperm storage organ. We use individual-based simulations and an analytical model to evaluate how selection on sperm size depends on promiscuity level and variation in sperm storage organ size (hereafter, female preference variation). Simulated species with high promiscuity showed stabilizing selection on sperm when female preference variation was low, and disruptive selection when female preference variation was high, consistent with the analytical model results. With low promiscuity (2-3 mates per female), selection on sperm was stabilizing for all levels of female preference variation in the simulations, contrasting with the analytical model. Promiscuity level, or mate sampling, thus has a strong impact on the selection resulting from female preferences. Further, for species with low promiscuity, disruptive selection on male traits will occur under more limited circumstances than many previous models suggest. Variation in female sperm storage organs likely has strong implications for intraspecific sperm variation in highly promiscuous species, but likely does not explain differences in intraspecific sperm variation for less promiscuous taxa.

evolutionary biology↗