# New insights into the putative XX/XY sex chromosomal system in blue‐eyed red‐fin pleco Hypostomus soniae (Siluriformes, Loricariidae)

## TL;DR

This study investigates the possible XX/XY sex system in blue-eyed red-fin pleco fish and finds evidence that challenges the hypothesis.

## Contribution

The study provides new cytogenetic evidence suggesting the XX/XY system may not exist in this species.

## Key findings

- The karyotype of H. soniae is 2n = 64 with size heteromorphism between sexes.
- Multiple NORs and CMA3-positive marks were found on acrocentric chromosomes.
- Tip-to-tip pairing in pachytene bivalents suggests absence of an XY pair with partial synapsis.

## Abstract

Blue‐eyed red‐fin pleco Hypostomus soniae (family Loricariidae) presented a putative sex system XX/XY in early stage. Aiming to explore the inter‐populational karyotypic variation and proposed emergence of the XX/XY system, we studied 13 H. soniae individuals (6 males, 7 females) from the Tapajós River. Mitotic karyotypes and meiotic cells were analysed using C‐banding, site verification Ag‐NOR (nucleolar organizing region), chromomycin A3 (CMA3) and fluorescent in situ hybridization with repetitive DNA probes (DNAr 18s, 5S, histone H1, H3, telomere). The synaptonemal complex on meiocytes was studied using immunodetection with anti‐structural maintenance of chromosomes protein 3, anti‐γH2AX and anti‐H3K9ac. The karyotype presented was 2n = 64, with clear size heteromorphism varying between male and female. The species presented multiple NORs colocalized with 18S and CMA3‐positive marks in two pairs of acrocentric chromosomes. The centromeric region of pairs 25 and 26 carries the repeats of histones H1 and H3. All the bivalents at pachytene exhibited tip‐to‐tip pairing, revealing the absence of an XY pair with partial synapsis. The synaptic behaviour of the putative sexual pair failed to corroborate the XX/XY system hypothesis in H. soniae. Further cytogenetic and molecular investigations are necessary to determine the proposed emergence of an XX/XY sexual system in H. soniae and the regulatory mechanisms underlying its atypical meiotic behaviour.