Sperm length divergence as a potential prezygotic barrier in a passerine hybrid zone

Cramer, Emily R. A.; Grønstøl, Gaute; Maxwell, Logan M.; Kovach, Adrienne I.; Lifjeld, Jan T.

doi:10.1002/ece3.7768

Cited by 3 publications

(3 citation statements)

References 63 publications

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“…Measurements of the midpiece and tail length were taken by several observers. While measurement repeatability was not directly assessed for the samples in this study, previous work in our group shows high measurement repeatability: 94%-98% for midpiece and 80%-95% for tail (Cramer et al, 2020;Cramer, Grønstøl, et al, 2021 ;Laskemoen et al, 2010). We assessed how mean light microscopy measures from the same individual males correlated with SEM-based length measurements (Supporting Information: Table S2).…”

Section: Light Microscopymentioning

confidence: 99%

Flagellum tapering and midpiece volume in songbird spermatozoa

Cramer

Grønstøl

Lifjeld

2022

Journal of Morphology

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In contrast to numerous studies on spermatozoa length, relatively little work focuses on the width of spermatozoa, and particularly the width of the midpiece and flagellum. In flagellated spermatozoa, the flagellum provides forward thrust while energy may be provided via mitochondria in the midpiece and/or through glycolysis along the flagellum itself. Longer flagella may be able to provide greater thrust but may also require stronger structural features and more or larger mitochondria to supply sufficient energy. Here, we use scanning electron microscopy to investigate the ultrastructure of spermatozoa from 55 passerine species in 26 taxonomic families in the Passerides infraorder. Our data confirm the qualitative observation that the flagellum tapers along its length, and we show that longer flagella are wider at the neck. This pattern is similar to mammals, and likely reflects the need for longer cells to be stronger against shearing forces. We further estimate the volume of the mitochondrial helix and show that it correlates well with midpiece length, supporting the use of midpiece length as a proxy for mitochondrial volume, at least in betweenspecies studies where midpiece length is highly variable. These results provide important context for understanding the evolutionary correlations among different sperm cell components and dimensions.

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Section: Light Microscopymentioning

confidence: 99%

Flagellum tapering and midpiece volume in songbird spermatozoa

Cramer

Grønstøl

Lifjeld

2022

Journal of Morphology

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“…We do not specifically report measurement repeatability, as measurement repeatability sets an upper bound on the other measures of repeatability reported here. Other studies have shown that repeatability of such measurements are high and measurement error low (Laskemoen et al 2007(Laskemoen et al , 2010, and that measurement repeatability by the same observer is high (Lifjeld et al 2016;Cramer et al 2021).…”

Section: Statisticsmentioning

confidence: 99%

“…Recent advances in this field, in combination with better phylogenetic reconstructions, have shown how sperm size and components may evolve at different rates in different clades and lineages (Omotoriogun et al 2016;Supriya et al 2016), and that the risk of sperm competition can be an important driver in the evolution of sperm traits (Eberhard 1996;Briskie et al 1997;Immler et al 2012; van der Horst and Maree 2014; Rowe et al 2015;Durrant et al 2020). Phenotypic divergences in sperm traits have been indicated as playing a role in speciation processes (Cramer et al 2016(Cramer et al , 2021 and in the delimitation of species (Lifjeld et al 2016). However, given the fact that birds represent one of the betterstudied groups of animals, with a wealth of natural history information available (Billerman et al 2020), it is noteworthy that there still is a large proportion of bird species whose sperm is yet undescribed or unknown.…”

Section: Introductionmentioning

confidence: 99%

Effects of fixatives and storage duration on avian sperm morphology

et al. 2022

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Sperm cells are variable both within and among species. To be able to accurately measure sperm cells and understand their function, it is important that sperm cells are preserved in a manner that maintains their structural integrity. Formalin is a widely used fixative and storage medium for sperm cells, but few studies have examined the effect of fixation and long-term storage on their morphological integrity. Ethanol is also a common fixation and storage agent for tissue samples, and here we examine if fixation and storage in formalin or ethanol alters sperm cell size and structural integrity. We found no significant effects of the fixation process on fresh sperm cells fixed in formalin or ethanol. Further, there were no consistent length changes in sperm cells stored in formalin or ethanol over a period of 227 days, or in sperm cells stored in formalin for three years. A comparison across 13–14 years of storage time showed a small but significant reduction in sperm cell length of 0.93%. Furthermore, sperm cells initially fixed in formalin remained quite stable in dry storage on glass slides for a minimum of six months (we found a mean reduction in sperm cell length of 0.18% after 6 months). The proportion of sperm cells with head damage was, however, much higher for samples stored in ethanol than for those stored in formalin. Overall, 70% of sperm cells had acrosome damage in ethanol versus only 3% in formalin. Finding intact sperm cells for measuring length, therefore, required greater effort in ethanol samples than in formalin samples. Our findings indicate that use of sperm cells from long-term storage for the study of sperm morphometrics is justified for either fixative, although formalin clearly preserves the sperm cells better.

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Sperm as a speciation phenotype in promiscuous songbirds

Lifjeld,

Cramer,

Leder

et al. 2024

Evolution

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Sperm morphology varies considerably among species. Sperm traits may contribute to speciation if they diverge fast in allopatry and cause conspecific sperm precedence upon secondary contact. However, their role in driving prezygotic isolation has been poorly investigated. Here we test the hypothesis that, early in the speciation process, female promiscuity promotes a reduction in overlap in sperm length distributions among songbird populations. We assembled a data set of 20 pairs of populations with known sperm length distributions, a published estimate of divergence time, and an index of female promiscuity derived from extrapair paternity rates or relative testis size. We found that sperm length distributions diverged more rapidly in more promiscuous species. Faster divergence between sperm length distributions was caused by the lower variance in the trait in more promiscuous species, and not by faster divergence of the mean sperm lengths. The reduced variance is presumably due to stronger stabilizing selection on sperm length mediated by sperm competition. If divergent sperm length optima in allopatry causes conspecific sperm precedence in sympatry, which remains to be shown empirically, female promiscuity may promote prezygotic isolation and rapid speciation in songbirds.

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Sperm length divergence as a potential prezygotic barrier in a passerine hybrid zone

Abstract: This is an open access article under the terms of the Creat ive Commo ns Attri bution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Cited by 3 publications

References 63 publications

Flagellum tapering and midpiece volume in songbird spermatozoa

Flagellum tapering and midpiece volume in songbird spermatozoa

Effects of fixatives and storage duration on avian sperm morphology

Sperm as a speciation phenotype in promiscuous songbirds

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