Microbial biomarkers as indicators of sperm viability in an insect

Abstract

Our understanding of microbial variation in male reproductive tissues is poorly understood, both regarding how it varies spatially across different tissues and its ability to affect male sperm and semen quality. To redress this gap, we explored the relationship between male sperm viability and male gut and reproductive tract microbiomes in the Pacific field cricket, Teleogryllus oceanicus. We selected cohorts of males within our populations with the highest and lowest natural sperm viability and characterized the bacterial microbiota present in the gut, testes, seminal vesicle, accessory glands and the spermatophore (ejaculate) using 16S ribosomal RNA gene metabarcoding. We identified bacterial taxa corresponding to sperm viability, highlighting for the first time an association between the host's microbial communities and male competitive fertilization success. We also found significant spatial variation in bacterial community structure of reproductive tissue types. Our data demonstrate the importance of considering the microbial diversity of both the host gut and reproductive tract when investigating male fertility in wildlife and potentially human clinical settings.

Keywords: reproductive microbiome; sexual selection; sperm competition.

Figure 1.

Alpha diversity indices (a) observed ASVs, (b) Chao1 index, (c) inverse Simpson’s index and (d) Shannon’s index by tissue type. Boxes cover the interquartile range (IQR) and the diamond inside the box denotes the median. Whiskers represent the lowest and highest values within 1.5 × IQR. Different lowercase letters signify significant differences in Tukey’s honest significant difference (HSD) post hoc tests.

Figure 2.

Principal coordinate analysis (PCoA) of reproductive tissue and gut samples (n = 138) using a Bray–Curtis distance matrix by sperm viability cohort (a) or tissue type (b). Axis 1 explains 20.6% of the variation while axis 2 explains 19.1%. Ellipses indicate 95% confidence intervals.

Figure 3.

Relative abundance of bacterial phyla (a,b) and genera (c,d) present across the male tissue types (a,c) or by sperm viability (b,d). Phyla and genera whose relative abundance was less than 1 and 4%, respectively, across all tissue types and viability levels were pooled. Different lowercase letters for tissue type signify significant differences in Tukey’s honest significant difference (HSD) post hoc tests. There was no significant difference between the sperm viability cohorts.

Figure 4.

Histogram (a) shows the magnitude of the observed effect due to each biomarker using a linear discriminant analysis (LDA) score for high (blue) or low (yellow) sperm viability with LEfSe. The log10 normalized abundance of each of these biomarkers is shown in a heatmap (b) for all samples (each column is a sample).