Chromosomal complements of some Atlantic Blennioidei and Gobioidei species (Perciformes).

A remarkable degree of chromosomal conservatism (2n=48, FN=48) has been identified in several families of Perciformes. However, some families exhibit greater karyotypic diversity, although there is still scant information on the Atlantic species. In addition to a review of karyotypic data available for representatives of the suborders Blennioidei and Gobioidei, we have performed chromosomal analyses on Atlantic species of the families Blenniidae, Ophioblennius trinitatis Miranda-Ribeiro, 1919 (2n=46; FN=64) and Scartella cristata (Linnaeus, 1758)(2n=48; FN=50), Labrisomidae, Labrisomus nuchipinnis (Quoy & Gaimard, 1824)(2n=48; FN=50) and Gobiidae, Bathygobius soporator (Valenciennes, 1837)(2n=48; FN=56). Besides variations in chromosome number and karyotype formulas, Ag-NOR sites, albeit unique, were located in different positions and/or chromosome pairs for the species analyzed. On the other hand, the heterochromatic pattern was more conservative, distributed predominantly in the centromeric/pericentromeric regions of the four species. Data already available for Gobiidae, Blenniidae and Labrisomidae show greater intra- and interspecific karyotypic diversification when compared to other groups of Perciformes, where higher uniformity is found for various chromosome characteristics. Evolutionary dynamism displayed by these two families is likely associated with population fractionation resulting from unique biological characteristics, such as lower mobility and/or specific environmental requirements.

Introduction

Although karyotypic characteristics for some families of marine fish are already known, information on groups of Perciformes is still significantly disproportionate. Among these, suborders Blennioidei and Gobioidei stand out because of the large number of species they represent.

Suborders Gobioidei, with 2,121 species, and Blennioidei with 732 species, are spread throughout the tropical zone, typically represented by small specimens with low mobility and the ability to withstand changes in temperature and salinity (Nelson 2006).

Species of Blennioidei and Gobioidei investigated (e.g. Cataudela et al.1973; Garcia et al.1987; Ene 2003) have shown sufficient chromosomal peculiarities for species discrimination and understanding of their evolutionary aspects. In some families, such as Blenniidae, Labrisomidae and Gobiidae, sharing cryptic morphological characteristics combined with poor knowledge of the biological characteristics for many species, contributes to the relative taxonomic inaccuracy of this group. As such, cytotaxonomic markers (Garcia et al. 1987; Caputo 1998; Caputo et al. 2001) and phylogenetic analyses based on molecular data (Wang et al. 2001; Thacker 2003; Gysels et al. 2004; Almada et al. 2005) have been increasingly used when assessing their kinship relations. Indeed, it has been suggested that phylogenetic analyses combine molecular and morphological data (Thacker 2003), as well as cytogenetic information. However, in light of the diversity in these groups, solid chromosome data are not yet sufficiently available, with only 7.5% of Bleniidae species and 4.5% of Gobioidei was karyotyped (Table 1). Despite the scarcity of data, a high degree of chromosomal polymorphism has been characterized among Gobiidae, primarily Robertsonian rearrangements (Caputo et al. 1999, Ene 2003), along with others such as tandemfusions and pericentric inversions (Giles et al. 1985; Thode et al. 1985; Amores et al. 1990).

Table 1.

Cytogenetic data for Blennioidei and Gobioidei (Perciformes).

Suborder/Family	Species	2n	Karyotype formula	FN	References
Blennioidei
Blenniidae	Aidablennius sphynx	48	4m+4sm+40a	56	Cano et al. (1982)
	Aidablennius sphynx	48	2st+46a	50	Cataudella and Civitelli (1975)
	Atrosalarias fuscus	48	48a	48	Arai and Shiotsuki (1973)
	Blennius ocellaris	48	2m+2st+44a	52	Vitturi et al. (1986)
	Blennius ponticus	48	16sm+10st+22a	74	Garcia et al. (1987)
	Blennius yatabei	48	6sm+12st+30a	66	Arai and Shiotsuki (1974)
	Coryphoblennius galerita	48	2m+12sm+34a	62	Garcia et al. (1973)
	Dasson trossulus	40	8m+32st/a	48	Arai and Shiotsuki (1974)
	Istiblennius enoshimae	48	2m+46a	50	Arai and Shiotsuki (1973)
	Istiblennius lineatus	48	48st/a	48	Arai and Shiotsuki (1974)
	Lipophrys canevai	48	8st+40a	56	Cataudella and Civitelli (1975)
	Lipophrys pholis	46	8m+8sm+30a	62	Garcia et al. (1987)
	Lipophrys trigloides	46	4m+4sm+10st+28a	64	Cano et al. (1982)
	Lipophrys trigloides	48	2m+6sm+18st+22a	74	Cataudella and Civitelli (1975)
	Lipophrys trigloides	48	2m+22sm+2st+22a	74	Garcia et al. (1987)
	Lipophrys trigloides	48	2m+6sm+18st+22a	74	Vitturi et al. (1986)
	Omobranchus elegans	42	10m+2sm+6st+24a	60	Arai and Shiotsuki (1974)
	Omobranchus punctatus	44	4m+40a	48	Arai (1984)
	Ophioblennius trinitatis	46	6m+12st+28a	64	Present study
	Parablennius incognitus (= Blennius incognitus)	48	4st+44a	52	Cano et al. (1982)
	Parablennius pilicornis (= Blennius pilicornis)	48	8st+40a	56	Catalano et al. (1985)
	Parablennius gattorugine	48	2m+4sm+42a	54	Vitturi et al. (1986)
	Parablennius pilicornis	48	48a	48	Brum et al. (1992)
	Parablennius sanguinolentus	48	12st+36a	60	Cataudella et al. (1973)
	Parablennius sanguinolentus	48	20sm+10st+18a	78	Garcia et al. (1987)
	Parablennius tentacularis	48	48st/a	48	Vasil’ev (1985)
	Parablennius tentacularis	48	1st+47a	49	Carbone et al. (1987)
	Parablennius tentacularis	47	1sm+46a	48	Carbone et al. (1987)
	Salaria fluviatilis	48	48st/a	48	Cataudella and Civitelli (1975)
	Salaria pavo	48	8st+40a	56	Cataudella et al. (1973)
	Salaria pavo	48	16sm+14st+18a	78	Garcia et al. (1987)
	Salaria pavo	48	2st+46a	50	Vasil’ev (1980)
	Salarias faciatus	48	48a	48	Arai and Shiotsuki (1973)
	Salarias luctuosus	48	48st/a	48	Arai and Shiotsuki (1974)
	Scartella cristata (= Blennius cristatus)	48	2st+46a	50	Vitturi et al. (1986)
	Scartella cristata	48	2sm+46st/a	50	Brum et al. (1995)
	Scartella cristata	48	4st+44a	52	Present study
Gobioidei
Clinidae	Clinithracus argentatus	48	2st+46a	50	Vitturi et al. (1986)
Labrisomidae	Labrisomus nuchipinnis	48	2sm+46a	50	Affonso (2000)
	Labrisomus nuchipinnis	48	2st+46a	50	Present study
Eleotridae	Dormitator latifrons	46	44m/sm+2st/a	90	Uribe-Alcocer et al. (1983)
	Dormitator maculatus	46	34m/sm+12st/a	80	Maldonado-Monroy et al. (1985)
	Dormitator maculatus	46	40m/sm+6st/a	86	Molina (2005)
	Dormitator maculatus	46	14m+28sm+2st+2a(♀) 13m+28sm+3st+2a(♂)	90	Oliveira and Almeida-Toledo (2006)
	Eleotrioides strigatus	44	2m+42st/a	46	Arai and Sawada (1974)
	Eleotris acanthopomus	46	46st/a	46	Arai and Sawada (1974)
	Eleotris picta	52	52a	52	Uribe-Alcocer and Diaz-James (1996)
	Eleotris pisonis	46	2m/sm+42st/a	46	Uribe-Alcocer and Diaz-James (1996)
	Eleotris pisonis	46	46a	46	Rocon-Stange (1992)
	Eleotris pisonis	46	46a	46	Molina (2005)
	Eleotris muralis	46	46a	46	Khuda-Bukhsh and Nayak (1990)
	Mogurnda mogurnda	46	6sm+40st/a	52	Arai et al. (1974)
	Mogurnda obscura	62	-	-	Nogusa (1960)
	Ophiocara porocephala	48	48a	48	Arai and Fujiki (1979)
	Oxyeleotris marmorata	46	2m+2sm+42a	50	Arai and Fujiki (1979)
Gobiidae	Aboma latipes	40	40a	40	Arai and Sawada (1974)
	Acanthogobius flavimanus	44	44st/a	44	Arai and Sawada (1974)
	Acanthogobius flavimanus	44	36st+8a	80	Arai and Kobayashi (1973)
	Acanthogobius flavimanus	44	10m/sm/st+34a	54	Arai and Sawada (1975)
	Acentrogobius pflaumi	50	48m/sm+2st/a	98	Nogusa (1960)
	Amblygobius albimaculatus	44	2m+42st/a	46	Nishikawa et al. (1974)
	Aphia minuta	44	44a	44	Caputo et al. (1999)
	Aphia minuta	43	42a+1st	42	Caputo et al. (1999)
	Aphia minuta	42	1m+1st+40a	44	Caputo et al. (1999)
	Aphia minuta	42	1M+1m+40a	44	Caputo et al. (1999)
	Aphia minuta	41	2M+1st+38a	44	Caputo et al. (1999)
	Apocryptes bato	46	24m+10sm+12a	80	Nayak and Khuda-Bukhsh (1987)
	Apocryptes lanceolatus	38	14m+22sm+2st	76	Nayak and Khuda-Bukhsh (1987)
	Awaous grammepomus	46	46st/a	46	Khuda-Bukhsh and Barat (1987)
	Awaous tajasica	46	46a	46	Stange and Passamani (1986)
	Bathygobius fuscus	48	48a	48	Arai and Sawada (1975)
	Bathygobius soporator	48	2m+46a	50	Brum et al. (1996)
	Bathygobius soporator	48	2m/sm+46a	50	Cipriano et al. (2002)
	Bathygobius soporator	48	2m+6st+40a	56	Present study
	Bathygobius stellatus	46	2st+44a	48	Vasil’ev (1985)
	Bathygobius stellatus	47	1sm+2st+43a	49	Vasil’ev (1985)
	Boleophthalmus boddaerty	46	46m/sm	92	Subrahmanyan (1969)
	Boleophthalmus glaucus	46	12m+20sm+2st+12a	80	Manna and Prasad (1974)
	Boleophthalmus pectinirostrus	46	46st/a	46	Arai and Sawada (1975)
	Bostrichthys sinensis	48	4m/sm+44a	52	Arai et al. (1974)
	Chaenogobius annularis	44	18sm+26st/a	62	Arai and Sawada (1975)
	Chaenogobius annularis	44	36m/sm+8a	80	Arai et al. (1974)
	Chaenogobius annularis	44	44a	44	Nogusa (1960)
	Chaenogobius castaneus	44	36m/sm/st+8a	80	Nishikawa et al. (1974)
	Chaenogobius isaza	44	12sm+32st/a	56	Arai and Sawada (1975)
	Chaenogobius urotaenia	44	-	-	Nogusa (1960)
	Chaenogobius urotaenia	42	14sm+28a	56	Yamada (1967)
	Chasmichthys dolichognatus	44	44st/a	44	Arai and Sawada (1975)
	Chaenogobius gulosus	44	44st/a	44	Arai and Sawada (1975)
	Chaenogobius gulosus	44	16m/sm/st+28a	60	Nishikawa et al. (1974)
	Ctenogobius criniger	50	34m/sm+6st+10a	90	Arai and Sawada (1974)
	Gillichthys mirabilis	44	12sm+32a	56	Chen and Ebeling (1971)
	Gillichthys seta	44	6m+14sm+24a	64	Chen and Ebeling (1971)
	Glossogobius fasciatopunctatus	44	10m+28sm+2st+4a	84	Fei and Tao (1987)
	Glossogobius giuris	46	46a	46	Rishi and Singh (1982)
	Gobiodon citrinus	44	2m+42st/a	46	Arai and Sawada (1974)
	Gobiodon citrinus	43	1m+42st/a	44	Arai and Sawada (1974)
	Gobiodon quinquestrigatus	44	44a	44	Arai and Fujiki (1979)
	Gobiodon rivulatus	44	44a	44	Arai and Fujiki (1979)
	Gobioides rubicundus	46	2m+26sm+10st+8a	84	Manna and Prasad (1974)
	Gobionellus shufeldti	48	48a (♀)	48	Pezold (1984)
	Gobionellus shufeldti	47	46a+1m (♂)	48	Pezold (1984)
	Gobiosoma macrodon	38	38a	38	Musammil (1974)
	Gobiosoma zebrella	38	38a	38	Musammil (1974)
	Gobius abei	46	-	-	Nogusa (1960)
	Gobius bucchichi	44	2sm+42a	46	Thode and Alvarez (1983)
	Gobius cobitis	46	46a	46	Caputo et al. (1997)
	Gobius cruentatus	46	2st+44a	48	Thode and Alvarez (1983)
	Gobius fallax	38	8m/sm+30a	46	Thode et al. (1988)
	Gobius fallax	39	7m/sm+32a	46	Thode et al. (1988)
	Gobius fallax	40	6m/sm+34a	46	Thode et al. (1988)
	Gobius fallax	40	7m/sm+33a	47	Thode et al. (1988)
	Gobius fallax	41	5m/sm+36a	46	Thode et al. (1988)
	Gobius fallax	42	4m/sm+38a	46	Thode et al. (1988)
	Gobius fallax	43	3m/sm+40a	46	Thode et al. (1988)
	Gobius niger	52	2m+4sm+16st+30a	74	Vitturi and Catalano (1989)
	Gobius niger	51	3m+4sm+16st+28a	74	Caputo et al. (1997)
	Gobius niger	50	4m+4sm+16st+26a	74	Caputo et al. (1997)
	Gobius niger	49	5m+4sm+16st+24a	74	Caputo et al. (1997)
	Gobius paganellus	48	2sm+46a	50	Caputo et al. (1997)
	Gobius similis	44	?		Nogusa (1960)
	Gobiusculus flavescens	46	6m/sm+40a	52	Klinkhardt (1992)
	Luciogobius grandis	44	?		Arai (1981)
	Luciogobius guttatus	44	?		Arai and Kobayashi (1973)
	Mesogobius batrachocephalus	30	16m+14a	46	Ivanov (1975)
	Neogobius cephalarges	46	46a	46	Vasil’ev (1985)
	Neogobius constructor	42	4m/sm+38a	46	Vasil’ev and Vasil'yeva (1994)
	Neogobius cyrius	36	structural polymorphism		Vasil’ev and Vasil'yeva (1994)
	Neogobius fluviatilis	46	46a	46	Vasil’ev (1985)
	Neogobius eurycephalus	32	12m+2sm+18a	46	Ene (2003)
	Neogobius eurycephalus	31	13m+2sm+16a	46	Ene (2003)
	Neogobius eurycephalus	30	14m+2sm+14a	46	Ene (2003)
	Neogobius gymnotrachelus	46	46a	46	Vasil’ev and Grigoryan (1992)
	Neogobius kessleri	46	46a	46	Vasil’ev (1985)
	Neogobius melanostomus	46	46a	46	Vasil’ev (1985)
	Neogobius rhodionovi	46	46a	46	Vasil’ev and Vasil'yeva (1994)
	Odontamblyops rubicundus	46	4m+16sm+26st/a	66	Arai and Sawada (1975)
	Padogobius martensi	46	1m+3sm+2st+40a	52	Cataudella et al. (1973)
	Parioglossus raoi	46	46st/a	46	Webb (1986)
	Periophthalmus cantonensis	46	18m+12sm+16st/a	76	Arai and Sawada (1975)
	Pomatoschistus lozanoi	37	3m+12sm+10st+12a	62	Webb (1980)
	Pomatoschistus microps	46	4m+16sm+20st+6a	86	Klinkhardt (1989)
	Pomatoschistus minutus	46	4m+16sm+16st+10a	82	Klinkhardt (1989)
	Pomatoschistus minutus	46	18sm+18st+10a	82	Klinkhardt (1992)
	Pomatoschistus norvegicus	32	10m+10sm+8st+4a	60	Webb (1980)
	Pomatoschistus pictus	46	22m/sm+12st+12a	80	Klinkhardt (1992)
	Proterorhinus marmoratus	46	46a	46	Rab (1985)
	Pterogobius elapoides	44	14sm+30st	88	Arai and Kobayashi (1973)
	Pterogobius zonoleucus	44	14sm+30st	88	Arai and Sawada (1975)
	Quietula guaymasiae	42	6m+4sm+32a	52	Cook (1978)
	Quietula y-cauda	42	42a	42	Cook (1978)
	Rhinogobius brunneus	44	44a	44	Nishikawa et al. (1974)
	Rhinogobius flumineus	44	44a	44	Arai and Kobayashi (1973)
	Rhinogobius giurinus	44	44a	44	Nishikawa et al. (1974)
	Rhodoniichthys laevis	42	16m/sm+26st	84	Arai et al. (1974)
	Sicyopterus japonicus	44	10m+10sm+24a	64	Arai and Fujiki (1979)
	Synechogobius hasta	44	2m+42st/a	46	Arai and Sawada (1975)
	Tridentiger obscurus	44	10m/sm+34a	54	Arai et al. (1974)
	Tridentiger trigonocephalus	44	28m/sm/st+16a	72	Arai et al. (1973)
	Tridentiger trigonocephalus	46	16sm+6st+24a	68	Fei and Tao (1987)
	Trypauchen vagina	46	12m+6sm+10st+18a	74	Khuda-Bukhsh (1978)
	Tukugobius flumineus	44	44a	44	Nadamitsu (1974)
	Zosterisessor ophiocephalus (= Gobius ophiocephalus)	46	46a	46	Vasil’ev (1985)
	Zosterisessor ophiocephalus (= Gobius ophiocephalus)	45	1st+45a	47	Vasil’ev (1985)
	Zosterisessor ophiocephalus	46	2m/sm+44a	48	Caputo et al. (1996)

The present study focuses on the karyotypic characterization of some Atlantic species of the families Blenniidae, Miranda-Ribeiro, 1919 and (Linnaeus, 1758), Labrisomidae, (Quoy & Gaimard, 1824)and Gobiidae, (Valenciennes, 1837), through conventional chromosomal analysis, characterization of nucleolar organizer regions (Ag-NORs) and the distribution pattern of C-positive heterochromatin (C-banding) in chromosomes, discussing evolutionary aspects.

Material and methods

A total of 25 specimens of (7♂, 4♀ and 14 indeterminate), 11 specimens of (4♂, 5♀ and 2 indeterminate), 13 specimens of (4♂, 4♀ and 5 indeterminate) and 12 specimens of , (5♂, 5♀ and 2 indeterminate) were used for chromosome analysis. specimens came from the coast of Rio Grande do NortePageBreakPageBreakPageBreakPageBreak (5°13'1.73"S; 35°9'57.85"W), northeastern Brazil (n=1), and the Saint Peter and Saint Paul (n=8) (00°55'02"N; 29°20'42"W) and Fernando de Noronha (n=16) (3°52'11"S; 32°26'13"W) archipelagos. The remaining specimens were collected on the coast of Rio Grande do Norte. Individuals were previously submitted to mitotic stimulation with compound attenuated antigens, for 24 to 48 hours (Molina 2001, Molina et al. 2010), anesthetized with clove oil (Eugenol) and sacrificed for the removal of anterior kidney fragments. Sexing of specimens was performed by macroscopic and microscopic examination of the gonads. Chromosome preparations were obtained from kidney PageBreakcells (Gold et al. 1990). Nucleolar organizer regions (NORs) were identified by stain with silver nitrate - Ag-NORs (Howell and Black 1980) and C-positive heterochromatin sites through C-banding (Sumner 1972).

Metaphase preparations were examined and photographed on an Olympus BX50 photomicroscope, using an Olympus DP70 digital camera system. Chromosomes were classified according to the position of the centromere in metacentrics (m), submetacentrics (sm), subtelocentrics (st) and acrocentrics (a) (Levan et al. 1964) and organized in order of decreasing size. The chromosome formula and FN (fundamental number or number of chromosomal arms) were established for each species, considering acrocentric chromosomes with a single arm and the remaining chromosomes exhibiting two arms.

Results

Cytogenetic analyses of Blenniidae species (Blennioidei)

showed 2n=46, with a chromosome formula equal to 6m+12st+28a (FN=64), irrespective of sex. Although chromosomes showed a gradual decline in size, the smallest acrocentric pairs corresponded to approximately one-third of the largest metacentric pairs. Nucleolar organizer regions are located in the terminal portions of the short arm on pair 9, the smallest subtelocentric pair. C-positive heterochromatin is discretely located in the centromeric/pericentromeric region of the chromosomes (Fig. 1a, b).

Figure 1.

Karyotypes underGiemsa staining a, c, e, g and C-banding b, d, f, h of ; a, b ; c, d ; e, f and ; g, h Ag-NOR-bearing chromosome pairs are highlighted.

showed 2n=48 chromosomes, with a chromosome formula equal to 4st+44a (FN=52). The karyotype also displays a gradual reduction in chromosome size. However, the largest chromosome pair exhibits only double the size in relation to the smallest karyotype pair. Ribosomal sites are located on the terminal portions of the short arms on chromosome pair 1. C-positive heterochromatin is also reduced and located in the centromeric regions of chromosomes (Fig. 1c, d).

Cytogenetic analyses of Labrisomidae and Gobiidae species (Gobioidei)

(Labrisomidae) showed 2n=48 chromosomes with a chromosome formula of 2st+46a (FN=50), showing relatively more differentiated size between the largest and smallest chromosomes of the karyotype. Nucleolar organizer regions are in the terminal portions of the long arms on pair 2, corresponding to the largest pair of acrocentric chromosomes. C-positive heterochromatin was showed in the centromeric/pericentromeric region of all chromosome pairs, in relatively conspicuous blocks (Fig. 1e, f).

(Gobiidae) also displayed the karyotype composed of 2n=48 chromosomes, but with the chromosome formula distinct from that of , specifically, 2m+6st+40a (FN=56). Size difference between the largest and smallestPageBreakPageBreak chromosomes of the karyotype was far less pronounced. Ribosomal sites were on the terminal portions of the short arms on chromosome pair 4. C-banding showed discrete heterochromatic regions in the centromeric regions of most chromosomes and telomeric regions of some acrocentric pairs (Fig. 1g, h).

Discussion

Though many perciform families display a conserved karyotype pattern, with 2n=48 acrocentric chromosomes, some groups demonstrate dynamic tendencies in relation to chromosome evolution (Molina 2007). Much of identifiable chromosome diversity is attributed to pericentric inversions, the most common mechanism of chromosome evolution in this order (Galetti et al. 2000, 2006).

Representatives of the suborder Blennioidei (e.g., Carbone et al. 1987) and Gobioidei (e.g., Arai and Sawada 1974, 1975; Thodeet al. 1988; Oliveira and Almeida-Toledo 2006) stand out for their greater karyotype variability and diversity. This includes species with conserved karyotyes and those that are highly diversified.

Within the Blennioidei, the Blenniidae, a monophyletic family, is divided into six tribes including Salariini and Parablenniini which, in turn, include the Atlantic species and respectively (Nelson 2006). Comparisons of mitochondrial DNA sequences in samples of Gill, 1860 collected throughout the Atlantic suggest that the genus consists of six distinct lineages. One of these corresponds to species found in the Pacific, while the rest are recorded in the biogeographic provinces of the Atlantic: Brazilian, Caribbean, Mid-Atlantic, Sao Tome and Azores/Cape Verde (Muss et al. 2001). Chromosome characteristics reported here for are the first for the genus, exhibiting 2n=46, 6m+12st+28a and FN=64. The relatively low diploid number and higher fundamental number in relation to the mean of other species of Blenniidae (Table 1), as well as the presence of large metacentric chromosomes, suggests pericentric inversion events and the occurrence of Robertsonian translocation involving two of its chromosome pairs. In turn, ,while also belonging to the family Blenniidae, has a distinct karyotype of 2n=48, 4st+44a and FN=52. Thus, differs from in that it contains an extra pair of chromosomes, lacks metacentric chromosomes and has different numbers of subtelocentric and acrocentric chromosomes in the karyotype. The karyotype of the population studied here differs from the karyotypes previously described for the coastal population of Rio de Janeiro (SE Brazil), with 2sm+46st/a (Brum et al. 1994), and the Mediterranean population, with 2st+46a (Vitturi et al. 1986). Nevertheless, despite the growing number of discordant karyotype descriptions between populations on the NE and SE coasts of Brazil, one cannot rule out that these differences may arise from the difficulty in precisely defining types of cryptic chromosomes in the karyotype of this species.

In spite of displaying relative diversity in chromosome structure, only 18.5% of Blennioidei species exhibit differences in the basal diploid number, 2n=48 chromosomes. As shown in table 1, diploid numbers for representatives of this suborder vary PageBreakbetween 2n=40, found in (Jordan & Snyder, 1902)(Arai and Shiotsuki 1974) and 2n=52 in Linnaeus, 1758 (Vitturi and Catalano 1989), but with a conspicuous modal value of 2n=48.

In contrast to Blennioidei, suborder Gobioidei shows much more dynamic karyotype evolution, demonstrating highly variable karyotype patterns, where the diploid number ranges from 2n=30 for (Kessler, 1874) (Ene 2003), to 2n=62 in (Richardson, 1844) (Nogusa 1960). Cytogenetic data for 95 species show that only 9.6% have 2n=48 chromosomes, whereas the highest frequencies observed correspond to 2n=46 in 40% of species investigated, and 2n=44 in 32% (Table 1). As such, both Gobioidei species studied here are included in the group showing 2n=48 chromosomes, with 2st+46a and FN=50 and with 2m+6st+40a and FN=56. Thus, differs from in the presence of metacentric chromosomes and different numbers of subtelocentric and acrocentric chromosomes in the karyotype.

Among chromosome rearrangements involved in karyotypic differentiation of Gobiidae, Robertsonian fusions stand out, and are likely the most common event in this group (Amores et al. 1990; Galetti et al. 2000). However, other more complex changes in karyotypic structure (Thode et al. 1988; Vitturi and Catalano 1989; Caputo et al. 1997; Caputo et al. 1999), as well as the presence of different sex chromosomes (e.g., Pezold 1984; Baroiller et al. 1999), can also be observed, corroborating the high dynamic evolution that characterizes suborder Gobioidei. It has been suggested that the baseline/ancestral karyotype for Gobiidae would consist of 2n=46 acrocentric chromosomes (Vasil’ev and Grigoryan 1993), from which an increase in bi-brachial chromosomes would characterize more derived karyotypes. Based on this proposal, (FN=56) would experience a greater number of structural rearrangements during its karyotypic evolution process in relation to (FN=50).

Location and frequency of Ag-NOR sites are efficient cytotaxonomic markers in many groups of fish (Caputo 1998). Among species of Gobiidae, at least six different arrangement patterns for nucleolar organizer regions have been identified (Fig. 2), which supports the occurrence of intense karyotypic diversification mechanisms in this group. Thus, Ag-NOR sites can be found (a) in the telomeric region on the short arm of a single pair of acrocentric chromosomes, as in Sarato, 1889 (Thode et al. 1983) and Linnaeus, 1758 (Caputo 1998); (b) in the telomeric region on the long arm of a single pair of acrocentrics, such as in (Pallas, 1814) (Caputo 1998); (c) in the interstitial/pericentromeric region on the long arm of a single pair of acrocentric chromosomes, as seen in (Pallas, 1814) (Ráb 1985) and Pallas, 1814 (Caputo 1998); (d) in the telomeric region on the short arm of a single subtelocentric pair, described in ;(e) in the interstitial/pericentromeric region on the long arm of a single metacentric pair, observed in (Ene 2003); and (f) in the telomeric regions on the short arms of two acrocentric chromosome pairs, recorded in (Fabricius, 1779) (Klinkhardt 1992).PageBreak

Figure 2.

Ag-NOR phenotypes a–f described in species of Gobiidae. Ag-NORs sites described in the karyotypes of Gobiidae species were found a in the telomeric region on the short arm of a single pair of acrocentric chromosomes b in the telomeric region on the long arm of a single pair of acrocentrics c in the interstitial/pericentromeric region on the long arm of a single pair of acrocentric chromosomes d in the telomeric region on the short arm of a single subtelocentric pair e in the interstitial/pericentromeric region on the long arm of a single metacentric pair and f in the telomeric regions on the short arms of two acrocentric chromosome pairs.

Few data are available on ribosomal sites for Labrisomidae. Ag-NORs in exhibit the phenotype (b) described above, in addition to both species of Blenniidae, and , which may suggest an ancestral condition for this location.

In contrast, other chromosome characteristics, such as C-positive heterochromatin distribution, may be more conserved. This occurs in several species of Percifomes where discrete blocks are preferentially located in the centromeric/pericentromeric regions of chromosomes (Molina 2007). This pattern is repeated in , and , as well as in some Gobiidae, such as , and (e.g. Caputo et al. 1997; Ene 2003). In ,in addition to centromeric/pericentromeric regions, heterochromatic sites are also observed in terminal regions of some chromosomes. This arrangement has already been described for other Gobiidae, including and , where pericentromeric and telomeric heterochromatic regions are distributed among almost all chromosomes (Amores et al. 1990; Caputo et al.1997).

Moreover, karyotypic diversity present in Gobioidei is increased by the occurrence of chromosome polymorphisms frequently observed in this group. This is particularly evident in several examples of intraspecific karyotypic variability, as well as polymorphisms involving different types of chromosome rearrangements, such as in (Vitturi and Catalano 1989; Caputo et al. 1997) and (Thode et al. 1988). Data obtained for the paedomorphic Gobiidae (Risso, 1810) also show variations in the diploid number and chromosome formula, resulting in five different cytotypes (2n=41–44 and FN=42-44) (Caputo et al. 1999). Similar karyotypic variability was reported in , where three specific cytotypes (2n=30, 31 and 32) were associated to the occurrence of centric fusions (Ene 2003). All these examples demonstrate clear chromosomal dynamism, with possible transitions to new karyotype patterns.

PageBreakIn fact, karyotypic diversity among Blennioidei and Gobioidei seems to accompany phyletic diversification of these groups. This is a result of vicariant factors (Pampoulie et al. 2004) and could be favored by their low dispersive potential (Fanta 1997), as well as ecological specificities that favor population fractionation in this family (Huyse et al. 2004). The present study also highlight the importance of ribosomal sites as effective chromosomal markers in the further cytogenetic studies in gobiids species.