DOI: 10.31196/huvfd.2002787 ISSN: 2146-717X

Characterizing Individual Variation in Frozen-Thawed Bull Sperm Heads Using Landmark- Based Geometric Morphometrics

Mustafa Yiğit Nizam, Ali Şimşek, Bülent Bülbül, Yasin Demiraslan, Murat Selcuk
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Accurate characterization of sperm-head morphology is essential for understanding morphological variation associated with bull fertility. However, conventional morphometric approaches largely rely on linear measurements and may overlook localized shape differences within the sperm head. This pilot study investigated whether landmark-based geometric morphometrics could detect subtle individual differences in frozen-thawed bull sperm-head morphology. Archived frozen semen samples from eight clinically and andrologically healthy, fertile bulls were evaluated. Following thawing and Eosin Y staining, sperm heads were photographed and 15 points were digitized along the sperm-head outline. Generalized Procrustes Analysis was used to remove variation related to position, orientation, and scale. Differences in centroid size and shape were assessed using Procrustes analysis of variance, while multivariate regression was used to evaluate and remove the allometric effect before Principal Component Analysis and Discriminant Function Analysis. Significant differences among individuals were detected for both centroid size and sperm-head shape (p<0.0001), and centroid size had a significant effect on shape (p<0.0001). After correction for allometry, the first four principal components explained 74.72% of residual shape variation. The main morphological differences were concentrated in the acrosomal region and acrosome-head transition and, to a lesser extent, at the head-midpiece junction. Despite considerable overlap among individuals in morphospace, discriminant analysis yielded an overall cross-validated correct classification rate of approximately 65%. These findings demonstrate that landmark-based geometric morphometrics can detect subtle, anatomically localized variation in morphologically normal frozen-thawed bull spermatozoa and provide complementary information beyond conventional morphometry.

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