Jukka Kekäläinen

Mature sperm are widely considered as transcriptionally inactive cells owing to the tight compaction of their chromatin with protamine. However, sperm cells have been demonstrated to retain RNA polymerase subunits, chromatin remodellers, transcription factors, and parts of their chromatin remain in a less condensed, nucleosome-bound configuration. Consequently, some studies have suggested that sperm may be transcriptionally more active than previously assumed. Here, we tested this possibility by exposing mature human sperm to follicular fluid (FF), a bioactive liquid surrounding the oocyte. Using RNA sequencing, we first observed that FF exposure alters the sperm transcriptome. To resolve whether the observed upregulation of selected candidate genes resulted from the transcriptional activation of these genes in sperm, we labelled newly synthesized RNAs with 5-ethynyl uridine. Our results suggest that FF may trigger de novo transcription of sperm CXCL8 and CXCL3 genes and indicate that some follicular-fluid-derived transcripts are selectively transferred into sperm. Collectively, these findings suggest that specific sperm genes may become transcriptionally active in the presence of biologically relevant stimuli, potentially preparing sperm for fertilization and early embryo development. Furthermore, we envisage that activated sperm genes can have novel implications for the deeper mechanistic understanding of sexual selection.

Sulphate input from industrial discharge, mining, and agricultural runoff can significantly alter biogeochemical functions in freshwater systems. However, the concentration‐ and time‐dependent effects of sulphate on geochemistry and microbial communities of lake sediments remain insufficiently characterized. Here, we incubated lake sediment microcosms amended with 1, 5, and 10 mM sulphate under suboxic conditions for 45 days and monitored geochemical and microbial community changes. Sulphate enrichment induced distinct, dose‐ and time‐responsive changes. Nitrate increased progressively with sulphate concentration (10 > 5 > 1 mM), reflecting treatment‐specific shifts in nitrogen dynamics under changing redox conditions within sedimentary nitrogen pools. Microbial diversity declined initially across sulphate treatments, peaked at 5 mM, and remained suppressed at 10 mM. Dominant taxa shifted from Geobacter spp. at 1 mM to Citrifermentans spp. and Sulfurospirillum spp. at 10 mM, reflecting redistribution of electron flow among competing iron, sulphur, and nitrogen associated metabolisms at higher sulphate concentrations. Functional predictions suggested enhanced iron respiration at lower sulphate (1 mM), whereas higher concentrations were associated with increased chemoheterotrophy and fermentation, alongside suppression of methanogenesis. Our findings demonstrate that increasing sulphate inputs restructure sedimentary microbial networks and reconfigure redox‐linked carbon and nitrogen cycling, suggesting ecosystem‐level biogeochemical responses to rising sulphate stress in freshwater catchments.

Freshwater ecosystems are under significant environmental stress due to warming and plastic pollution. However, our understanding of their combined effects on primary producers is scarce. We investigated the effects of plain spherical polymethylmethacrylate (PMMA) nanoparticles (NPs) and the acute temperature increases on the growth and fatty acid content of the freshwater green algae Pseudokirchneriella sp. over a 5‐day exposure period. The experiment was conducted at three NP concentrations (0.05, 0.5, and 5 mg/L) and two temperature levels (20°C and 25°C). We analysed algal organic matter (AOM) produced during the experiments. Higher temperature stimulated cell growth at lower NP concentrations, but not at the highest NP concentration. Fatty acid composition was affected by temperature but not by NPs. At 20°C, the fractions of low, intermediate, and high molecular weight (MW) AOM had a higher tryptophan‐like fluorescence, pointing to a higher protein‐like content. Humic‐like fluorescence of low MW AOM fractions was higher at higher temperature. At 25°C, the fluorescence response increased slightly with increasing NP concentrations. The findings suggest that temperature has a greater effect on altering fatty acid composition and AOM chemistry than NPs.

Major histocompatibility complex (MHC) genes are known to mediate mate choice both at the individual and gamete level. However, it has remained unclear how different episodes of MHC-associated mate choice interact and contribute to the total selection on MHC genes. Here, we clarified this interaction in humans by performing a full-factorial experiment where 10 females first ranked the attractiveness and intensity of the body odours of 11 males. Then we studied whether female odour preferences in these same 110 male-female combinations predicted sperm performance in the presence of follicular fluid (sperm-stimulating female reproductive fluid). When analyzing the total MHC similarity (including classical and non-classical MHC genes) of the male-female combinations, we found that females preferred the body odours of MHC-similar males, but that sperm motility was positively affected by the MHC dissimilarity of the male-female combinations. No associations were found for classical MHC genes only. Furthermore, odour preferences were negatively associated with sperm motility at the end of the follicular fluid treatment. Together, our results indicate that individual and gamete-level mate choice processes may act in opposing directions and that the most attractive males are not necessarily the most optimal partners at the post-copulatory level. Finally, our findings suggest that gamete-mediated mate choice may have a definitive role in disfavouring genetically incompatible partners from fertilizing oocytes.

Numerous studies have shown that a healthy reproductive tract microbiota is crucial for successful reproduction and that its composition is influenced by various environmental and host factors. However, it is not known whether the reproductive microbiota is also shaped by the major histocompatibility complex (MHC), a family of genes essential to differentiate ‘self’ from ‘non-self’ peptides to initiate an adaptive immune response. We tested the association between the follicular fluid microbiome and MHC genes in 27 women. Women with higher MHC diversity had a higher microbiome diversity, characterized by bacteria commonly associated with vaginal dysbiosis. Women with similar MHC genes were also similar in their microbiome composition, indicating that MHC composition may be a key factor in determining the bacterial assemblage in the reproductive tract. Finally, the composition of the follicular fluid microbiome was similar to the vaginal microbiome, suggesting that numerous bacteria of the vagina are true inhabitants of the follicular fluid or that vaginal microbiota contaminated the follicular fluid microbiota during transvaginal collection. Collectively, our results demonstrate the importance of host genetic factors in shaping women's reproductive microbiota and they open the door for further research on the role of microbiota in mediating MHC-related variation in reproductive success.

Anodonta anatina, a widely distributed Eurasian freshwater mussel, has dramatically declined and listed as endangered species in IUCN Red List in some parts of the Europe. To better understand its basic biology and possible sex-dependent effects of parasitism on reproduction, we investigated the reproductive traits, trematode parasitism and growth between November 2020 and October 2021. Our data from a Finnish population indicates male-biased sex ratio and the gonadal development lasting throughout the year with a bradytictic brooding pattern. Gonadosomatic index of both sexes peaked in May and declined in June. Mature oocytes were smaller sized than those of previously studied unionids, fecundity was fairly low, and the maximum predicted lifespan was 14 years. Gonads were infected with digenean trematode parasites, Rhipidocotyle campanula being the dominant species. Interestingly, trematode infection rate was higher in females than in males. In July, when the water temperature reached its annual maximum, the trematode prevalence also peaked, which was accompanied with 16% of mussels being sterile. The trematode parasitism was thus negatively associated with reproduction, that might become even more affected if parasitism increases along with the predicted climate warming, which should be considered in possible future conservation and aquaculture efforts of this species.

Threatened fish populations worldwide are relying on stock supplementation by hatchery‐reared fish. Although stocking with non‐native fish may lead to hybridization, thereby disrupting local adaptations, it could also improve the adaptive potential of small populations by increasing variability in essential behavioral traits. In this study, we crossbred the critically endangered Finnish landlocked salmon with a geographically close Baltic anadromous salmon population to compare boldness‐related behavioral traits among the crossing groups. We studied postrelease exploration in seminatural streams and tested stress tolerance of the fish, as reflected by their response during swimming trials and commencement of feeding after the trials. In the stress tolerance experiments, we compared fish with different rearing backgrounds to gain insight into environmentally induced variability in stress tolerance. When compared with the anadromous salmon, the landlocked salmon showed more active postrelease exploration and higher stress tolerance. The hybrids displayed intermediate behaviors compared with the purebred salmon, indicating heritability of these traits. The landlocked salmon showed less variable exploration than the hybrids with anadromous salmon as the maternal strain, but we did not find rearing‐dependent responses in stress tolerance. In summary, our findings suggest that hybridization could help in reversing likely domestication‐related negative behavioral traits in the landlocked salmon.

Infertility is currently regarded as a male or female-dependent pathologic condition. However, emerging evidence suggests that this may be an incomplete view as it misses the important fact that the gametes of some males and females often have higher compatibility than others. In this article, I present recent evidence demonstrating that female reproductive tract secretions in humans mediate “mate choice” at the level of the gametes, and this way likely bias fertilization toward immunologically or otherwise compatible males. A better understanding of this evolutionary selection process may open up novel possibilities for the future development of more personalized infertility diagnostics and treatments.

Cryptic female choice (CFC) is commonly assumed to act only in polyandrous mating systems, which allows females to bias fertilization towards the sperm of particular males. However, accumulated evidence has demonstrated that sperm show significant phenotypic and genotypic variation also within single ejaculates, which have important consequences for offspring phenotype and fitness. Here, I argue that these neglected sources of intra‐male sperm variation often allow CFC to act also within individual males and facilitate fertilization bias towards genetically compatible (or otherwise preferred) sperm haplotypes. In this article, I explain prerequisites for within‐male CFC, the criteria for demonstrating it and summarize accumulated evidence for this emerging selection process. Then, I evaluate prevalence of within‐male CFC and review its potential evolutionary consequences. The aim of this article is to broaden the current definition of CFC by demonstrating that CFC has potential to act in all mating systems, in both internally and externally fertilizing species. Incorporation of the within‐male CFC concept into the current models of sexual selection may provide novel insights into the deeper understanding of selective factors driving the evolution of mating systems and reproductive proteins. Finally, within‐male CFC towards particular sperm haplotypes may increase our understanding of non‐Mendelian inheritance.

Seminal plasma (SP) plays a crucial role in reproduction and contains a large number of proteins, many of which may potentially modify sperm functionality. To evaluate the effects of SP identity and its protein composition on human sperm function, we treated the sperm of several males with either their own or multiple foreign SPs in all possible sperm–SP combinations (full-factorial design). Then we recorded sperm motility and viability in these combinations and investigated whether the sperm performance is dependent on sperm and SP identity (or their interaction). Finally, we studied whether the above-mentioned sperm traits are affected by the abundance of three SP proteins, dipeptidyl peptidase-4 (DPP4), neutral endopeptidase (NEP), and aminopeptidase N (APN). The identity of the SP donor affected sperm swimming velocity, viability, and the proportion of hyperactivated sperm, but males’ own SP was not consistently more beneficial for sperm than foreign SPs. Furthermore, we show that sperm performance is also partly affected by the interaction between sperm and SP donor. Finally, we found that DPP4 and NEP levels in SP were positively associated with sperm swimming velocity and hyperactivation. Taken together, our results highlight the importance of seminal plasma as a potential source of biomarkers for diagnostics and therapeutic interventions for male-derived infertility.