RESULTS

We compiled 125 papers that included cytogenetic data on Loricariidae species. These studies comprised a diversity of 234 species, here included those identified as “sp.”, prope, “aff.”, “cf.”, “n.sp.”, and “L” as distinct species, from 48 genus. Hypostominae was the subfamily most represented with 142 species karyotyped, followed by Loricariinae (54 spp.), Hypoptopomatinae (34 spp.), and Delturinae and Rhinelepinae with 2 spp. each, with absence of data for Lithogeninae . The Hypostomus genus was the most represented with 26 valid species karyotyped, in a total of 159 records when including all populational data and those identified as “sp.”, prope, “aff.”, “cf.”, “n. sp.”, and “L”. The geographical coordinates plotted into maps show a distribution of species cytogenetically investigated in Brazil, Argentina, and Paraguay, comprising six distinct river basins (Fig. 2). Although species from Ecuador and Venezuela were also compiled, those papers do not present geographical coordinates of the sample sites.

Diploid number varied from 2n = 33 in males of Rineloricaria teffeana (Steindachner, 1879) (Marajó et al., 2022) to 2n = 96 in Hemipsilichthys sp. (Kavalco et al., 2004, 2005). The fundamental number ranged from 34 in R. teffeana (Marajó et al., 2022) to 142 in Hypostomus topavae (Godoy, 1969) (Kamei et al., 2017), and the simple distribution of Ag-NOR/18S rDNA was the most common with 269 records, against 110 records of multiple sites. Seven sex chromosomes systems were described for 31 Loricariidae species, the simple XX/XY, XX/X0, and ZZ/ZW, and the multiples X 1 X 1 X 2 X 2 /X 1 X 2 Y, XX/XY 1 Y 2, ZZ/ZW 1 W 2, and Z 1 Z 1 Z 2 Z 2 /Z 1 Z 2 W 1 W 2. B chromosomes were found in five species, varying from 1B (e.g., de Souza et al., 2009) to 3B chromosomes (e.g., Porto et al., 2010). Complete results were compiled in Tab. 1.

Subfamily/ GenusSpeciesLocality XimbaúvaLatitudeLongitudeBasin2nNFAgNOR - 18S MultipleKF 8m + 10sm + 18sm + 32aSCSReference/ Observation Hypostomus ancistroidesstream, Ourizo-IvaÍ68♀ ♂Endo et al. (2012) ancistroidesPiquiri River, Brazil68♀ ♂Multiple14m + 14sm + 8st + 32a Bueno et al. (2013) ancistroidesÁgua Boa stream, Mundo Novo, MS, Brazil68♀ ♂116Multiple14m + 24sm + 10st + 20aFernandes et al. (2012) ancistroidesDourado stream, Mundo Novo, MS, Brazil68♀ ♂116Multiple10m + 22sm + 16st + 20aFernandes et al. (2012) ancistroidesDourado stream, Mundo Novo, MS, Brazil68♀ ♂120Multiple14m + 16sm + 22st + 16aTraldi et al. (2013)ancistroides Hortelã stream, Botucatu, SP, Brazil 22°56'28.9"S48°35' 03.2"W68♀ ♂Multiple10m + 20sm + 10st + 28aPansonato-Alves et al. (2013) ancistroides Piquiri River, Nova Laranjeiras, Brazil 24°56'54.0"S52°35' 49.0"W68♀ ♂Multiple14m + 14sm + 8st + 32aBueno et al. (2014)ancistroidesMonjolinho River, São Carlos, SP, Brazil68♀ ♂102Multiple16m + 18sm + 34st/aLorscheider et al. (2015) cf. heraldoi Zawadzki, Weber & Pavanelli, 2008Mogi Guaçu River, Pirassununga, SP, BrazilMogi Guaçu72♀ ♂Simple6m + 6sm + 26st + 34aMartinez et al. (2011)cf. plecostomus (Linnaeus, 1758)Severo stream, Brazil9°54' 30.8"S56°03'33.9"WAmazon68♀ ♂120/ 121Multiple14m + 24sm + 14st + 16a ♂, 15m + 24sm + 14st + 15a ♀ZZ/ZWOliveira et al. (2015) cf. topavaeCarrapato stream, Penápolis, SP, BrazilParaná80♀ ♂Multiple6m + 8sm + 42st + 24aMartinez et al. (2011) cf. wuchereri (Günther, 1864)Mutum River, Jequié, BA, Brazil13°43'18.0"S39°51'20.0"WContas76♀ ♂104Simple10m + 18sm + 48st/aBitencourt et al. (2011b)cf. wuchereri Una River, Valença, BA, Brazil 13°21'55.0"S39°04'35.0"WRecôncavo Sul76♀ ♂104Simple10m + 18sm + 48st/aBitencourt et al. (2011b)cochliodon Kner, 1854Iguaçu River, Brazil64♀ ♂Simple12m + 16sm + 16st + 20a Bueno et al. (2014) cochliodonIguaçu River, Foz do Iguaçu, PR, Brazil25°38'53.0"S54°27' 28.0"W64♀ ♂Simple12m + 16sm + 16st + 20aRubert et al. (2016)cochliodonPiraputanga River, Cáceres, MT, Brazil16°03'33.0"S57°40'33.0"WParaguai64♀ ♂100Multiple16m + 20sm + 28st/aRubert et al. (2016) commersoni Valenciennes, 1836Iguaçu River, Brazil68♀ ♂Multiple12m + 14sm + 14st + 28a Bueno et al. (2013) commersoniLake of Ney Braga hydroeletric plant, Mangueririnha, PR, Brazil68♀ ♂100Multiple12m + 12sm + 8st + 36aMaurutto et al. (2012) commersoniPiquiri River, Nova Laranjeiras, Brazil24°56'54.0"S52°35'49.0"W68♀ ♂Multiple12m + 14sm + 14st + 28aBueno et al. (2014)commersoniIguaçu River, Foz do Iguaçu, PR, Brazil25°38'53.0"S54°27' 28.0"W68♀ ♂Multiple12m + 14sm + 14st + 28aBueno et al. (2014) commersoniIguaçu River, PR, Brazil26°15'01.1"S51°06'10.7"WParaná68♀ ♂106Multiple12m + 12sm + 14st + 30aLorscheideret al. (2018) derbyi (Haseman, 1911) Iguaçu River, Curitiba, PR, Brazil 66♀ ♂82Multiple6m + 10sm + 20st + 30aMaurutto et al. (2012) derbyiIguaçu River, PR, Brazil26°15'01.1"S51°06'10.7"WParaná68♀ ♂102Simple12m + 12sm + 10st + 34aLorscheideret al. (2018) faveolus Zawadzki, Birindelli & Lima, 2008Taquaralzinho River, MT, Brazil64♀ ♂Simple18m + 8sm + 22st + 16a Bueno et al. (2013) faveolusTaquaralzinho River, Barra do Garças, MT, Brazil15°40'42.0"S52°17'52.0"W64♀ ♂Simple18m + 8sm + 22st + 16aBueno et al. (2014) goyazensis (Regan, 1908)Vermelho River, GO, Brazil72♀ ♂ Simple10m + 16sm + 10st + 36aAlves et al. (2006) hermanni (Ihering, 1905)Piquiri River, PR, Brazil72♀ ♂Multiple10m + 8sm + 32st + 22aBueno et al. (2013) hermanniPiquiri River, Nova Laranjeiras, Brazil24°56'54.0"S52°35' 49.0"W72♀ ♂Multiple10m + 8sm + 32st + 22aBueno et al. (2014) hermanniPiracicaba River, Piracicaba, SP, Brazil22°43'07.0"S47°39' 19.0"W72♀ ♂98Simple8m + 18sm + 46st/aRubert et al. (2016)na, PR, BrazilSubfamily/ GenusSpeciesLocalityLatitudeLongitudeBasin2nNFAgNOR - 18SKFSCSReference/ Observation Hypostomus tapijara Oyakawa, Akama & Zanata, 2005Ribeira de Iguape River, Registro, SP, Brazil66♀ ♂118Multiple14m + 24sm + 14st + 14aTraldi et al. (2013) tietensis (Ihering, 1905)PiraÍ River, Brazil23°22' 22.0"S47°22' 13.0"WParaná72♀ ♂108Simple8m + 8sm + 20st + 36aAnjos et al. (2020) tietensisPiraÍ River, Brazil23°22' 22.0"S47°22' 13.0"WParaná72♀ ♂109Simple8m + 8sm + 20st + 36aPaula et al. (2022)Mogi Guaçu topavae (Godoy, 1969)River, Pirassununga, SP,80♀ ♂104Multiple8m + 16sm + 56st/a Artoni, Bertollo (1996). Reported as Hypostomus sp. EBrazilMogi Guaçu topavaeRiver, Pirassununga, SP,80♀ ♂104Multiple8m + 16sm + 56st/aLorscheider et al. (2015)Brazil topavae Piquiri River, Nova Laranjeiras, PR, Brazil 80♀ ♂14m + 10sm + 26st + 30aBueno et al. (2012) topavaePiquiri River, PR, Brazil80♀ ♂Simple14m + 10sm + 26st + 30aBueno et al. (2013) topavaePiquiri River, Nova Laranjeiras, PR, Brazil24°56'54.0"S52°35' 49.0"WParaná80♀ ♂Simple14m + 10sm + 26st + 30aBueno et al. (2014) topavaeKeller River, Brazil23°38' 25.9"S51°51' 32.8"W80♀ ♂142Multiple14m + 30sm + 18st + 18aKamei et al. (2017) unae (Steindachner, 1878)76♀ ♂10m + 20sm + 46st/aAnjos et al. (2020) Lasiancistrus schomburgkii (Günther, 1864)Massangana River, Brazil9°80′48″S*63°08′90″W*Amazon54♀ ♂108Simple28m + 16sm + 10stMariotto et al. (2019)sp.Cachoeira River, Brazil14°64′66″S*52°35′50″W*Tocantins-Araguaia54♀ ♂108Simple28m + 16sm + 10stMariotto et al. (2019) Megalancistrus sp. Cuiabá River, Brazil 15°62′90″S*56°08′70″W*Paraguai52♀ ♂104Simple28m + 16sm + 8stMariotto et al. (2019) parananus (Peters, 1881) Piquiri River, Nova Laranjeiras, PR, Brazil 52♀ ♂Simple18m + 24sm + 10stBueno et al. (2018)Camarapi River, Panaqolus sp.Portel, PA,52♀ ♂Simple24m + 18sm + 10st/aAyres-Alves et al. (2017)Brazil tankei Cramer & Sousa, 2016Xingu River, Brazil52♀ ♂104Simple6m + 38sm + 8stFerreira et al. (2021) armbrusteri Lujan, Hidalgo & Stewart, 2010Xingu River, PanaqueAltamira, PA,52♀ ♂Simple26m + 20sm + 6st/aAyres-Alves et al. (2017)BrazilGorgulho daarmbrusteriRita, Altamira,52♀ ♂Simple26m + 20sm + 6st/aAyres-Alves et al. (2017)PA, BrazilAraguaia River,cf. nigrolineatusBarra do Garças,52♀ ♂Simple26m + 20sm + 6st/aArtoni, Bertollo (2001) Peckoltia cavatica Armbruster & Werneke, 2005PA, Brazil52♀ ♂Simple38m /sm + 14stPety et al. (2018) multispinis (Holly, 1929)PA, Brazil52♀ ♂Simple28m /sm + 24stPety et al. (2018) oligospila (Günther, 1864)PA, Brazil52♀ ♂Multiple38m /sm + 14stPety et al. (2018) sabaji Armbruster, 2003PA, Brazil52♀ ♂Multiple38m /sm + 14stPety et al. (2018) sp. 1Jari riverJari River, Monte Dourado, PA, Brazil03°18'14.9"N52°03'29.3"W52♀ ♂102Multiple44m /sm + 6st + 2a + 1BSouza et al. (2009)Jari River, sp. 2Jari riverMonte Dourado,03°18'14.9"N52°03'29.3"W52♀ ♂10232m /sm + 18st + 2aSouza et al. (2009)PA, Brazilsp. 3 JarumãAbaetuba, PA, Brazil1°42'41.9"S48°51'45.9"W52♀ ♂Simple46m /sm + 6stSilva et al. (2021)sp. 4 CaripetubaAbaetuba, PA, Brazil1°37' 23.5"S48°55'33.0"W52♀ ♂Multiple40m /sm + 12stSilva et al. (2021) vittata (Steindachner, 1881) Xingu River, Altamira, PA, Brazil 03°12'4"N52°12' 41.7"W52♀ ♂102Simple16m + 20sm + 14st + 2aSouza et al. (2009)vittataPA, Brazil52♀ ♂Simple32m /sm + 18st + 2aPety et al. (2018) Pseudacanthicus leopardus (Fowler, 1914)Xingu River, BrazilXingu - Amazon52♀ ♂104Simple18m + 34sm Santos da Silva et al. (2022b) sp.Xingu River, BrazilXingu - Amazon52♀ ♂104Simple18m + 34smSantos da Silva et al. (2022b) spinosus (Castelnau, 1855)Tocantins River, BrazilTocantins-Araguaia52♀ ♂104Simple18m + 34sm Santos da Silva et al. (2022b) Pterygoplichthys ambrosettii (Holmberg, 1893)Preto River, Mirassolândia, SP, Brazil52♀ ♂Simple16m + 24sm + 8st + 4a Artoni et al. (1999 b). Reported as Liposarcus anisitsiambrosettiiTietê River, Botucatu, SP, Brazil52♀ ♂Simple28m + 12sm + 8st + 4a Alves et al. (2006). Reported as Liposarcus anisitsiMT, Brazil

DISCUSSION

The origin of the chromosomal diversity of Loricariidae is attributed to both ecological and molecular factors. Small and isolated populations, in addition to the low vagility, are the main ecological characteristics that have allowed the fixation of chromosomal rearrangements in Loricariidae, as suggested for Farlowella (Marajó et al., 2018), Harttia (Sassi et al., 2023), and Rineloricaria (Rosa et al., 2012) . The diploid number 2n = 54 is often considered the ancestral diploid number for the family, since it is observed in most Loricariidae subfamilies and present in other Siluriformes (Artoni, Bertollo, 2001) . However, there is no consensus on the matter particularly considering the new karyotypic description of basal taxa within subfamilies. Takagui et al. (2020) in a review of Loricariinae karyotypes argue that although predominant, 2n = 54 should not be considered the basal diploid number for the family because multiple divergences in the microstructure of karyotypes within the same 2n are recurrently seen throughout the family. Notably, the 2n range in Loricarioidei suggest that other numbers rather than 2n = 54 can be considered the plesiomorphic ones, as Astroblepidae presents 2n = 52–54, Scoloplacidae 2n = 50, Callichthyidae 2n = 40–134, and Trichomycteridae 2n = 32–62 (reviewed by Conde-Saldaña et al., 2018). Beyond that, the Diplomystidae (Siluroidei) presents 2n = 56 chromosomes (Campos et al., 1997; Oliveira, Gosztonyi, 2000), which might also suggest that as the ancestral 2n.

We plotted the 2n range per subfamily into the main molecular phylogenetic reconstructions of Loricariidae (Fig. 3) and 2n = 54 is the most widespread, being conserved in Rhinelepinae and present in all other subfamilies. Regardless matter whether 2n = 54 or 2n = 56 is the ancestral diploid number, it is noteworthy that chromosomal evolution in Loricariidae is complex. Considering the lower number of karyotyped species when compared to the valid richness, 234 spp. with cytogenetic data against 1,051 valid species (Fricke et al., 2023), it is difficult to establish the evolutionary pathways that led to the observed variability. Notably, while the stability is restricted to 2n in some genera, with high divergence in karyotype structure, as the 2n = 58 in Farlowella, 2n = 54 in Corumbataia, and 2n = 52 in Pterygoplichthys, stable 2n and karyotype is also observed, as the 2n = 52 (26m +20sm+6st/a) in two species of Panaque . Our compilation reveals that processes of ascending and descending dysploidy (i.e., the increase or decrease of 2n while preserving the genomic content) were frequent in most subfamilies but Rhinelepinae, which may exhibit the constant 2n = 54 as a symplesiomorphic trait.

Despite the 2n conservation in Rhinelepinae, variations in the karyotype structure are observed within and between species. Populations of Rhinelepis aspera Spix & Agassiz, 1829 from the same river differ in the karyotype formula (Artoni, Bertollo, 2001; Endo et al., 2012), suggesting that pericentric inversions played an important role in the chromosomal evolution of the species. Such mechanism is not restricted to Rhinelepinae but also observed in other loricariids, such as Ancistrus (Mariotto et al., 2009), Loricariichthys (Takagui et al., 2014), and Rineloricaria (Rosa et al., 2012; Primo et al., 2018). In addition, the multiple Ag-NOR observed in Pogonopoma wertheimeri (Steindachner, 1867) (Artoni, Bertollo, 2001) when compared to the simple distribution in other Rhinelepinae species suggest that other mechanisms are also important for the chromosomal evolution in the group. Notably, numeric and structural polymorphisms are frequently observed in loricariids, for example in the multiple karyomorphs of Rineloricaria pentamaculata Langeani & de Araujo, 1994 (Glugoski et al., 2023), and the presence of B chromosomes in Harttia longipinna Langeani, Oyakawa & Montoya-Burgos, 2001 (Blanco et al., 2012). This chromosomal diversity was probably generated by a combination of rearrangements that include Robertsonian fusions and fissions, paracentric and pericentric inversions, and translocations (Artoni, Bertollo, 2001; Kavalco et al., 2004; Ziemniczak et al., 2012).

Although rare in most fish species, being observed in only about 5% of Teleostei species (Arai, 2011; Sember et al., 2021), our compilation shows that Loricariidae species carry seven out of the nine known sex chromosome systems observed among fishes. Ancistrus was demonstrated to harbor the largest diversity of sex chromosomes with six of the seven recognized systems for the family distributed in 18 species, corresponding to about 23% of the valid species (Neuhaus et al., 2023). The simple XX/XY was the most predominant in the genus, recorded in A. maximus de Oliveira, Zuanon, Zawadzki & Rapp Py-Daniel, 2015 (Oliveira et al., 2010; Favarato et al., 2016), Ancistrus cf. dubius (Mariotto et al., 2011), Ancistrus sp. 1 Quianduba River (Silva et al., 2022, 2023), Ancistrus sp. 1 Maracapucú River (Santos da Silva et al., 2023), Ancistrus sp. 1 Ilha do Capim (Santos da Silva et al., 2023), Ancistrus sp. Catalão (Favarato et al., 2016), Ancistrus sp. L2 (Prizon et al., 2017), Ancistrus sp. L3 (Prizon et al., 2017), and Ancistrus sp. Purus (Oliveira et al., 2010; Favarato et al., 2016). Additionally, two multiple sex chromosome systems that were not observed in any other Loricariidae are recorded in Ancistrus: ZZ/ZW1 W 2 in A. clementinae Rendahl, 1937 (Nirchio et al., 2023), and Z 1 Z 1 Z 2 Z 2/Z1 Z 2 W 1 W 2 (Oliveira et al., 2008; Favarato et al., 2016). On other hand, Harttia harbor the highest number of multiple sex chromosome systems when compared to the number of valid species, with six occurrences representing about 25% of valid species (compiled in Sassi et al., 2021): XX/XY1 Y 2 in H. carvalhoi Miranda Ribeiro, 1939, H. intermontana Oliveira & Oyakawa, 2019, and Harttia sp. 1 (Centofante et al., 2006; Deon et al., 2020); X1X1X2X2/X1X2Y in H. duriventris Rapp Py-Daniel & Oliveira, 2001, H. punctata Rapp Py-Daniel & Oliveira, 2001, and H. villasboas Oyakawa, Fichberg & Rapp Py-Daniel, 2018 (Blanco et al., 2014; Sassi et al., 2020), in addition to putative simple XX/XY in H. rondoni Oyakawa, Fichberg & Rapp Py-Daniel, 2018 and H. torrenticola Oyakawa, 1993 (Deon et al., 2020; Sassi et al., 2020). The Loricariidae diversity of sex chromosomes was originated by rearrangements that include translocations (Blanco et al., 2014), centric fissions (Sassi et al., 2023); centric fusions (Centofante et al., 2006), and pericentric inversions (Artoni et al., 1998), including the combination of distinct rearrangements especially in the origin of multiple sex chromosome systems (Oliveira et al., 2008; Deon et al., 2022). Sex chromosomes in Loricariidae seems to have independent origins, but further research is required to explore the genomic content of those sex chromosomes and its origin. Indeed, there is a recognized lack of information regarding the effects of environmental cues and molecular/gene mechanisms in sex determination of Neotropical fishes (Fernandino, Hattori, 2019).

Most Loricariidae species present a single 18S rDNA/Ag-NOR site, which is also considered a plesiomorphic character in the group (Artoni, Bertollo, 2001; Kavalco et al., 2004), and the standard distribution in most vertebrates (Sochorová et al., 2018). Notably, such region in loricariids is involved in several chromosomal rearrangements, including the origin and differentiation of sex chromosomes, being considered evolutionary breakpoint regions in Ancistrus, Harttia, and Rineloricaria (Glugoski et al., 2018; Deon et al., 2022). Size heteromorphism in the Ag-NOR site is also common, probably because of unequal crossing-over between homologs (Takagui et al., 2020). According to our review, some genus conserved the simple Ag-NOR locus in all analyzed species to date (here included only those genera with more than one species karyotyped), namely as Baryancistrus, Corumbataia, Farlowella, Harttia, Hisonotus, Lasiancistrus, Loricaria, Loricariichthys, Neoplecostomus, Panaqolus, Panaque, Pareiorhina, Pseudacanthicus, Pterygoplichthys, and Scobinancistrus . On other hand, the multiple distribution of Ag-NOR seems to be conserved only in Hypancistrus, while other genus as Ancistrus, Hypostomus, and Rineloricaria present both simple and multiple distributions on chromosomes.

Although distributed throughout the Neotropical region, there is a predominance of cytogenetic studies in Loricariidae species from Brazil (Fig. 2). Few studies were conducted in other countries that includes Argentina, Ecuador, Paraguay, and Venezuela. Notably, those in Argentina and Paraguay were mostly restricted to the frontier region with Brazil. When accounting the Brazilian territory, is also notable that some regions are poorly represented in cytogenetic studies, especially the northeast in which at least nine states have not been included in the cytogenetic samplings. In addition, the Guianas Shield and Western Amazon regions are largely recognized as neglected regions in biogeographical and evolutionary studies (Cassemiro et al., 2023), also with little or absent cytogenetic information for Loricariidae . Despite the regional sampling gap problem, Loricariidae diversity is still insufficiently represented by cytogenetic studies. Our compilation recorded 234 species assessed by cytogenetic studies, that in comparison to the 1,051 valid species (Fricke et al., 2023), represents 22.26% of the family species richness. The diversity of genus assessed by cytogenetic studies was the highest in Hypoptopomatinae (42.1%), followed by Rhinelepinae (28.5%), Hypostominae (27.2%), Loricariinae (24.4%), Delturinae (20%), and Lithogeninae (0%). Besides Lithogeninae that do not have any species karyotyped to date, the subfamily Delturinae has only one genus and two unidentified species karyotyped: Hemipsilichthys sp. Paraitinga River (Kavalco et al., 2004, 2005), and Hemipsilichthys n. sp. Patos River (Alves et al., 2005). We suggest that further cytogenetic studies focus on expand the sampling in the northeast Brazil, the Western Amazon, the Guianas Shield, and other Neotropical countries, in addition to evaluate a more representative portion of the diversity.