# Seasonal Expression of Glucose Transporter 5 (GLUT-5) Protein in the Testes of Roundleaf Bats in Thailand

## TL;DR

This study shows how fructose transport and testicular changes in roundleaf bats support seasonal sperm production.

## Contribution

The study reveals seasonal expression patterns of GLUT-5 and its isoforms in testes of wild bats, linking them to reproductive cycles.

## Key findings

- GLUT-5 is strongly expressed in developing sperm cells but absent in supporting cells.
- Two GLUT-5 isoforms show distinct seasonal expression patterns.
- Testicular structure changes with peak germinal epithelium height in July.

## Abstract

Bats have unique reproductive strategies where sperm production and storage are aligned with seasonal conditions. This study examined how bats use fructose, an important energy source, by analyzing a sugar-transporting protein in the testes of three roundleaf bat species in Thailand during the breeding season. The protein was strongly expressed in developing sperm cells but absent in supporting cells, and its levels varied across months, reaching a peak in August during active spermatogenesis. Structural differences in the sperm-producing tissue were also found, with greater height and tubule size at the beginning of the season. Two forms of the protein were identified, each showing distinct patterns of expression, suggesting potential contributions to reproductive processes. These results highlight how the seasonal regulation of sugar transport and testicular architecture supports sperm maturation in bats. Such findings improve our understanding of reproductive biology in wild mammals and may offer insights into mechanisms of fertility regulation relevant to other species.

Bats have unique reproductive strategies that are closely related to testicular metabolic adaptations, such as prolonged sperm storage. This study examined the expression of glucose transporter 5 (GLUT-5), a fructose-specific member of the facilitative glucose transporter family, in the testes of roundleaf bats (Hipposideros spp.) collected from various locations in Thailand during their active reproductive season (July to September) and explored its association with biometric traits. To assess GLUT-5’s localization and expression levels, testicular tissues from 50 adult males representing Hipposideros larvatus, Hipposideros armiger, and Hipposideros lekaguli species were analyzed using immunohistochemistry and Western blotting. Strong GLUT-5 immunoreactivity was observed in the cytoplasm of spermatocytes, spermatids, and spermatozoa, while weak staining was seen in spermatogonia. No GLUT-5 expression was detected in Leydig or Sertoli cells. Staining intensity varied significantly by month, with the highest levels observed in August (p < 0.05), exceeding those in July and September. Western blotting identified two GLUT-5 isoforms (55 and 100 kDa), with relative intensities that changed across the reproductive timeline. In parallel, morphometric analysis revealed that the height of the germinal epithelium and the diameter of the seminiferous tubules were significantly greater in July than in August and September, reflecting peak spermatogenic activity. These findings suggest that the seasonal regulation of fructose transport, along with changes in testicular architecture, may support testicular function and sperm maturation. The differential expression of GLUT-5 isoforms may reflect their distinct roles in body growth, reproductive maturation, and seasonal testicular activity in Hipposiderid bats.