Abstract
Lignobrycon myersi is an endemic fish species from a few coastal rivers in northeastern Brazil. Based on molecular evidence, Lignobrycon myersi and genera Triportheus Cope, 1872, Agoniates Müller & Troschel, 1845, Clupeacharax Pearson, 1924 and Engraulisoma Castro, 1981 were placed in the family Triportheidae. In the present work, we report the first cytogenetic data for Lignobrycon myersi to test the hypothesis that Lignobrycon and Triportheus are closely related. Studied specimens presented 2n=52 with 28 metacentric (m), 18 submetacentric (sm) and six subtelocentric (st) chromosomes for males and 27 m, 19 sm and 6 st for females, characterizing a ZZ/ZW sex chromosome system. The Z chromosome corresponds to the largest chromosome in karyotype while the W is about 50% smaller than the Z and largely heterochromatic. Terminal nucleolus organizer regions, GC-rich sites and 18S rDNA signals were detected on pair 14. However, additional 18S rDNA sites were observed in the W chromosome. The 5S rDNA was mainly detected on long arms of pair 7. The apparent synapomorphic chromosomal traits of Triportheus and Lignobrycon myersi reinforce their close phylogenetic relationship, suggesting that the ZZ/ZW chromosome system in both genera has arisen before cladogenic events.
Highlights
Lignobrycon myersi Miranda-Ribeiro, 1956 is a small characin fish characterized by a compressed body with keeled coracoids, adapted to swim near the surface
L. myersi is characterized by the occurrence of differentiated sex chromosomes of ZZ/ZW type, being the Z chromosomes equivalent to the first and largest chromosomal pair
The karyotypic macrostructure of L. myersi is similar to that reported in Triportheus in as much as both genera share 2n = 52 biarmed chromosomes and a differentiated ZZ/ZW sex chromosome system (Table 1)
Summary
Lignobrycon myersi Miranda-Ribeiro, 1956 is a small characin fish (about 11 cm in length) characterized by a compressed body with keeled coracoids, adapted to swim near the surface. The type-locality of L. myersi is located in the Almada river basin, a costal drainage in Bahia (Castro and Vari 1990). 10 years later, this species was collected in the nearby Contas river basin in Bahia (Castro and Jucá-Chagas 2008). Because of its narrow geographic range, associated with intensive environmental degradation (deforestation, pollution and impoundment), L. myersi is currently listed in the IUCN Red List of Threatened Species of Brazil (Castro and Jucá-Chagas 2008). Phylogenetic studies using DNA sequences of two mitochondrial and three nuclear genes revealed that this monophyletic group should be expanded and elevated to a family status (Triportheidae), including the following genera of tetras or freshwater sardines: Agoniates Müller & Troschel, 1845, Clupeacharax Pearson, 1924, Engraulisoma Castro, 1981, Triportheus and Lignobrycon (Oliveira et al 2011)
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