Evidence of an XX / XY sex chromosome system in the fish Dormitator maculatus ( Teleostei , Eleotrididae )

The fish Dormitator maculatus has a chromosomes number of 2n = 46, females having a karyotype of 14 M, 28 SM, 2 ST and 2A and males 13 M, 28 SM, 3 ST and 2A. The presence of a heteromorphic pair in the males and a corresponding homomorphic pair in the females suggest the occurrence of an XX/XY sex chromosome system in D. maculatus. The putative X chromosome has a pericentromeric C-band positive segment and the putative Y chromosome a C-band positive short arm.

The family Eleotrididae is comprised of about 35 genera and 150 species inhabiting tropical and subtropical areas worldwide (Nelson, 1994).Fish of this family are mainly marine but some species live in brackish and freshwater (Nelson, 1994).The subfamily Eleotridinae is represented in the Neotropical region by six genera, including the genus Dormitator (Kullander, 2003).The present paper describes the karyotypes, nucleolus organizer regions (NORs) and distribution of C-band positive segments of Dormitator maculatus and reports the occurrence of a putative case of an XX/XY sex chromosome system in this species.
Seven males and five females Dormitator maculatus from the Itinga River, Mongaguá, São Paulo state, Brazil were karyotyped.Mitotic chromosome preparations were obtained from kidney and gill tissues using the air-drying technique of Bertollo et al. (1978).Chromosome morphology was determined on the basis of arm ratio, as proposed by Levan et al. (1964) and chromosomes were classified as metacentric (M), submetacentric (SM), subtelocentric (ST) or acrocentric (A).Silver staining of nucleolar organizer regions (Ag-NORs) followed the technique proposed by Howell and Black (1980) and C-banding was performed as described by Sumner (1972).
We found a D. maculatus chromosome number of 2n = 46, with females having a karyotype of 14 M, 28 SM, 2 ST and 2A (Figure 1a) and males 13 M, 28 SM, 3 ST and 2A (Figure 1b).The silver staining of the chromosomes revealed that the Ag-NORs were terminally located on the short arms of a larger SM pair (pair 10) (Figure 1a).Several faintly stained C-band positive segments were visible on many autosomes, mainly at the centromeric or pericentromeric position and distributed over the short arms of some chromosomes (Figure 2).The putative X chromosome had a pericentromeric C-band positive segment (Figure 2a) and the putative Y chromosome a heterochromatic short arm (Figure 2b).
Cytogenetic studies conducted in eleven species of Eleotrididae show that the diploid number ranges from 2n = 44 to 48 among ten species investigated (Klinkhardt et al., 1995). However, Nogusa (1955) reported the occurrence of 2n = 62 chromosomes in the Eleotrididae species Mogrunda obscura.An interesting characteristic of the members of Eleotrididae is the common occurrence of many uniarmed chromosomes, thus differing from the karyotype of Dormitator maculatus that has mainly biarmed chromosomes.A recent cytogenetic investigation of specimens of Awaous strigatus (Eleotrididae) from Belém (Pará state, Brazil) suggested the occurrence of an X 1 X 2 Y sex chromosome system in this species (I.L. Souza, personal communication).
The presence of a heteromorphic pair in the males and a corresponding homomorphic pair in the female D. maculatus suggest the occurrence of an XX/XY sex chromosome system in this species.The morphological difference observed between the homologues chromosomes of the heteromorphic pair was probably due to the occurrence of a pericentric inversion, changing a primitive M chromosome from a homomorphic pair to an ST chromosome.Ohno (1974) considered the occurrence of a chromosome inversion to be the first step in the evolution of sex chromosomes, and recent studies have shown that this type of chromosome rearrangement is very important (if not the most important) rearrangement in species evolution (Livingstone and Riesenberg, 2003;Navarro and Barton 2003).Further studies including additional chromosome banding techniques will be very useful to confirm the presence of an XX/XY sex chromosome system in Dormitator maculatus.

Figure 1 -
Figure 1 -Giemsa stained karyotypes of Dormitator maculatus.(a) Female; (b) Male.The Ag-NOR-bearing chromosome pair is shone in the in-