The Unique Karyotype of Henochilus wheatlandii, a Critically Endangered Fish Living in a Fast-Developing Region in Minas Gerais State, Brazil

Abstract

Henochilus wheatlandii, the only species of this genus, is critically endangered and was considered extinct for over a century. The rediscovery of this fish in 1996 made it possible to study its phylogenetic relationships with other species in the subfamily Bryconinae. The aim of this study was to characterise the karyotype of H. wheatlandii. Standard staining, C-positive heterochromatin and nucleolar organiser region (NOR) banding, chromomycin A3 staining, and fluorescent in situ hybridisation (FISH) using 5S rDNA and 18S rDNA probes were conducted on nineteen specimens collected in the Santo Antonio River, a sub-basin of the Doce River in Ferros municipality, Minas Gerais State, Brazil. Henochilus wheatlandii shared the same diploid number and chromosome morphology as other species of Bryconinae. However, its heterochromatin distribution patterns, NOR localisation, and FISH patterns revealed a cytogenetic profile unique among Neotropical Bryconinae, emphasizing the evolutionary uniqueness of this threatened species.

Cytogenetic and Molecular Data Demonstrate that the Bryconinae (Ostariophysi, Bryconidae) Species from Southeastern Brazil Form a Phylogenetic and Phylogeographic Unit

“…is similar to that previously reported in H . wheatlandii [ 13 ], corroborating a second pattern that diverged from other Neotropical Bryconinae such as B . cephalus , B .…”

“…Karyotypes composed of 2 n = 50 metacentric, submetacentric, and subtelocentric chromosomes characterize the Bryconinae [ 13 , 22 , 23 , 25 – 27 , 51 ] and Salminus species [ 27 , 52 , 53 ]. The close phylogenetic relationship between Brycon spp.…”

“…Consequently, the loss of equilocality is a synapomorphy that characterizes the southeastern coastal bryconins, whereas the presence of heterochromatic block in the telomeric region of the first pair is an autapomorphy in H . wheatlandii [ 13 ].…”

“…Few studies on Neotropical freshwater fish species have been conducted to test phylogeographic hypotheses with cytogenetic marks [ 10 , 13 , 15 , 16 ]. Most cytogenetic studies on Neotropical fishes aimed detecting cryptic species and chromosomal evolution in large taxonomic groups.…”

“…A third, divergent pattern occurs in the coastal bryconin H . wheatlandii , which has a pair of NORs located in a pair of subtelocentric chromosomes, heterochromatic blocks in subtelocentric chromosomes, and non-equilocal heterochromatin in its first chromosome pair [ 13 ]. Thus, based on the cytogenetic differences between H .…”