Morphological diversification between Perciformes groups was, in many cases, not accompanied by pronounced karyotype diversification (Molina et al. 2002; Motta-Neto et al. 2011c). This has been reported in several families, including Haemulidae, which exhibits extensive numerical and structural chromosome conservatism among its species (Motta-Neto et al. 2011a, b, c). Nevertheless, a number of families are characterized by chromosomal diversification, either in regard to diploid value or aspects of karyotype macrostructure (Caputo et al. 1996; Araújo et al. 2010).
In contrast with other fish families (Motta-Neto et al. 2011a), cytogenetic and morphological analyses indicated that high morphological differentiation among Selene species was also accompanied by some discernible chromosome changes, primarily diploid number variation and structural changes in one pair of chromosomes. Comparatively, S. brownii and S.
setapinnis exhibited greater morphological similarity between them. However, the karyotype of S. brownii maintains more basal characteristics of the order Perciformes, such as 2n = 48 acrocentric chromosomes, simple ribosomal sites, and reduced heterochromatin (Brum 1996). The karyotype of S.
setapinnis diverges from S. brownii in that it displays a lower diploid value (2n = 46) and large metacentric pair, suggesting a clear Robertsonian translocation events as the origin of the pair.
Ribosomal sites, located proximally to the centromere on the long arm of this pair and identified by Ag-NOR and mapping of 18S rDNA sequences, are compatible in size and position with those found in the conserved karyotype of S. brownii, which reinforced this hypothesis. Thus, S.
brownii and S.
setapinnis are completely distinct with regard to their cytogenetic patterns. Involvement by an ancestral chromosome pair, bearing NOR sites, in the formation of a single metacentric pair in S. setapinnis suggests a more derived condition for this species in relation to S. brownii. Unlike S.
brownii and S.
setapinnis, the chromosome bearing NOR sites in S. vomer is subtelocentric, indicating the occurrence of pericentric inversion in the formation of this pair, exclusive to this species. In conjunction, these results suggest chromosome rearrangement is favored by the presence of ribosomal sites. In fact, there are indications that heterochromatin congruent to NORs may play a role in evolutionary rearrangements involving NOR-bearing chromosomes (Fujiwara et al. 1998).
In the family Carangidae, approximately 85 % of species display 2n = 48 chromosomes. Of these, 30 % have karyotypes formed exclusively by acrocentric chromosomes (Chai et al. 2009), representing a basal characteristic for Perciformes. As such, numerical and structural alterations in this pattern are indicative of more derived karyotypes (Molina 2007), as in S.
setapinnis and S. vomer. On the other hand, presence of bibrachial chromosomes demonstrates evolutionarily more dynamic karyotypes in Carangidae when compared with some other marine Perciformes families, such as Chaetodontidae and Sciaenidae (Galetti et al. 2006; Accioly and Molina 2008).
Among primary karyotype diversification events, pericentric inversions play an important role in the family Carangidae. However, chromosome polymorphisms related to Robertsonian translocations in Trachurus and Seriola (Vitturi et al. 1986) indicate these occurrences also contribute to karyotype diversification in Carangidae and may become established in some species, as observed in S. setapinnis.
In light of the extensive karyotypical conservatism presented by some groups of Perciformes, NORs may be inefficient cytotaxonomic markers for some species (Molina and Galetti 2004). However, NORs located in chromosomes involved in rearrangements, as in Carangidae (Caputo et al. 1996; Sola et al. 1997; Rodrigues et al. 2007; present study), make them potentially effective taxonomic and population approaches for this group. Although species analyzed contained a nucleolar organizer pair as a specific marker, the genus Selene displayed several conservative traits in its karyotypes. These include a large number of acrocentric chromosomes, simple 18S rDNA sites not syntenic with 5S-rRNA genes, GC-rich regions coincident with NORs, 5S rDNA sites situated in chromosomal pairs corresponding to the presence of low heterochromatic content primarily in pericentromeric regions. These characteristics have been identified as basal to Perciformes (Galetti et al. 2006; Molina 2007) and indicative of significant chromosome conservatism with low evolutionary dynamics. (Motta-Neto et al. 2011a, b, c). Similarly to NORs, 5S rDNA sites have proven to be phylogenetically discriminatory markers between several species and populations of marine fish (Motta-Neto et al. 2011a, b). Some cases have identified possible participation by 5S sites in chromosome rearrangements (Molina and Galetti 2002). Nevertheless, among Selene species, 5S sequences have demonstrated a conserved condition with low evolutionary dynamics.
Mapping of telomeric sequences has shown chromosomal fusion points in some fish species (Phillips and Reed 1996; Fontana et al. 1998). Selene brownii, S. setapinnis and S. vomer sequences (TTAGGG)n are exclusively located in the terminal portions of chromosomes. In S. setapinnis (2n = 46), the reduced diploid number observed in the ancestral condition (2n = 48) is attributed to centric fusion. In such cases, acrocentric/telocentric chromosomes, which are now united by their centromeres, may maintain fragments of their telomeric DNAs at the points of the fusion, with the occurrence of interstitial telomeric sites (ITS). However, several centric fusion events did not indicate the presence of ITS, as observed in the large metacentric pair of S. setapinnis, likely due to loss of these sequences during fusion (Slijepcevic 1998). Nevertheless, the 18S rDNA site in this chromosomal pair is adjacent to the point of fusion, that is, proximal to the centromere. A similar situation was recorded in fish from the genus Chromis, where 5S sites were contiguous to chromosome fusion regions and no ITS sites were detected (Molina and Galetti 2002). Similarly, decondensed 5S rDNA sites, interspaced by ITS sites, are indicative of chromosome fusions (Rosa et al. 2012).
Morphologically, Carangidae are characterized by a compressed, though highly variable, body shape, whereas species from the genera Decapterus and Trachurus have slender bodies, and those from Selene exhibit taller bodies (Gushiken 1988; Reed et al. 2002). As such, three body patterns are found in the genus Selene: (1) steep head profile with elongated dorsal and anal fins, seen in S. vomer, (2) head profile rounded on the top and very steep, with short dorsal and anal fins, present in S. brownii and S. setapinnis and, (3) and intermediary profile to the previous two, found in S. osterdii (Smith-Vaniz 1984). Although these body characteristics appear to indicate substantial derivation among carangids, no study has established how morphological evolution with in the group occurred. Phylogenetic inferences, as yet unconfirmed, based on Cit B sequences (Reed et al. 2001), suggest that the short dorsal and anal fins, such as those found in S. brownii and S. setapinnis, are derived. However, this hypothesis is not substantiated by cytogenetic data, since S. brownii displays a karyotype with characteristics considered the most basal among the three species analyzed here.
As such, in light of the apparent inconsistency between cytogenetic and molecular data, aspects of body shape were quantified in order to determine possible relationships between the species. Comparison of the body pattern of Selene species, polarized by members of the genus Oligoplites (O.
palometa), which are phylogenetically more basal and Caranx (C. lugubris), a more modern group (Reed et al. 2002), enabled quantification of the morphological amplitude between these species (Table 1). Morphological patterns are notably different among Selene and representatives of the genera Oligoplites and Caranx. Selene brownii and S.
setapinnis show greater morphological proximity in species analyzed. In other words, S. setapinnis exhibits a more divergent pattern in relation to the basal external group, followed by S. brownii, while S. vomer differs more from these two species and is less divergent from O.
palometa and C. lugubris. Thus, if these patterns are an accurate representation of phylogenetic relationships with in the family, this would suggest that chromosome rearrangements occur independently between species and, as such, would not be valid phylogenetic markers.
Morphological patterns are often indicators of the lifestyle of a species (Karr and James 1975; Wainwright and Reilly 1994). In fact, strong associations have been observed between basic shape and ecological function (Winemiller 1992). Morphological analyses in Selene suggest its steeper head profile is a derived characteristic, effectively separating S. brownii and S. setapinnis from S. vomer, which exhibits a less steep profile. However, significant differences in the anteroposterior axis are also discriminatory for the species. Therefore, the oval and deeper body characteristic of S. setapinnis, followed by a more oblong pattern in S. brownii, is sequentially enhanced in S. vomer, C. lugubris, and O. palometa. In association with these features, differences in the caudal peduncle were also detected in the five species investigated. This is a discriminating characteristic in juvenile phases of Selene, as well as other species (Winans and Nishioka 1987; Lima-Filho et al. 2006).
Data obtained in this study appear to indicate a lack of evolutionary synchrony between morphological changes and karyotype alterations in Selene species, which has also been suggested for other families of Perciformes (Molina et al. 2002; Motta-Neto et al. 2011c). However, this is not a universal occurrence, since distinct situations have also been observed among Perciformes. Thus, integrating cytogenetic approaches with body patterns has shown combined variation in both parameters for other species, such as Bathygobius soporator (Lima-Filho et al. 2012). In addition to corroborating molecular evidence identifying S. brownii and S. setapinnis as distinct taxa (Reed et al. 2001, 2002), cytogenetic aspects and body shape patterns shown here also highlight divergences of these two species with the Atlantic lookdown, S. vomer. The reduced species diversity observed in the genus Selene, combined with morphological peculiarities and low general variation of karyotypes among its species, makes this taxon an effective and interesting evolutionary model within the family Carangidae, deserving of more detailed investigation.