Our phylogenetic analyses provided molecular support for the recognition of three main clades in Serrasalmidae, congruent with previous studies (Ortí et al. 2008; Cione et al. 2009; Thompson et al. 2014; Mateussi et al. 2020; Favarato et al. 2021). The previous phylogenies currently subdivide Serrasalmidae into two subfamilies: Colossomatinae [ Clade I] and Serrasalminae (split into the tribes Myleini [CladeII] and Serrasalmini [Clade III]) (Mateussi et al. 2020). When evaluating the ancestral karyotype in our topology, it was revealed that during the cladogenesis of this family, there were two distinct events of chromosomal rearrangements. The first one, leading to a descending dysploidy to n = 27 (2n = 54) in the subfamily Colossomatinae (Clade I), and the other, an ascending dysploidy in the subfamily Serrasalminae with Myleini and Serrasalmini tribes (Clade II and III), showing n = 29 (2n = 58), n = 30 (2n = 60), 31 (2n = 62) and n 32 (2n = 64) respectively. Regarding the first diverging lineage of the family, some studies have suggested n = 27 as the most plesiomorphic karyotype, due to its presence in older groups, such as the genera Mylossoma, Brachypomus, Colossoma and Piaractus (Nakayma et al. 2012). The chromosome number 2n = 54 was also detected in other representatives of the order Characiformes, such as the families Anostomidae and Prochilodontidae, which have a high degree of chromosomal conservation (Vicari et al. 2006; Aguilar and Galleti, 2008). However, our ancestral chromosomal reconstruction did not allow us to reinforce this hypothesis.
Throughout our chromosome number reconstruction, it was possible to identify ascending dysploid events (increasing the chromosome numbers) in the most derived genera, such as Mylesinus, Myleus and Myloplus, which present 2n = 58. This increase becomes accentuated in representatives of the most diverse genera, Pygopristis, Pygocentrus and Serrasalmus with 2n = 60, 62 and 64. The differential morphology of karyotypes with more derived chromosome numbers, such as an increase in acrocentric chromosomes, indicates that chromosomal fission rearrangements drive the karyoevolution of the Serrasalmidae family (Nakayama et al. 2012). Centric fissions lead to karyotype diversity within a population, consequently increasing the probability of genetic isolation and speciation (Perry et al. 2004). Indeed, these processes appear to have been one of the main mechanisms in cladogenetic events in this family.
Our biogeographic reconstruction and molecular dating also allowed us to understand the phylogenetic relationships in a temporal context and discuss geographic distribution across space and time. Regarding the geographic distribution, most of the representatives of Serrasalmidae occur in the Amazon basins, which suggests that this region is the center of origin of the group. On the other hand, much is discussed about the evolutionary origin of the family, which has often been controversial. Some authors suggest an older origin, around 66 − 56 Ma, during the Middle Paleocene, but with the beginning of its diversification around 45 Ma, in the late Eocene (Thompson et al. 2014; Burns and Sidlauskas, 2019). Another study pointed to a younger age, between 42 − 38 Ma (Kolmann et al. 2021). Our analyses point to a scenario between 48 to 38 Ma, within the threshold of previous studies, associated with the uplift of the Andes (Armijo et al. 2015). In this context, the diversification scenario has been congruent with these studies (Burns and Sidlauskas, 2019; Kolmann, 2021).
The cladogenetic events that separate the three major clades are related to the late Eocene and early Oligocene (38 to 30 Mya) in our study, which coincides with the great division of the West-East Amazon drainage, with the origin of the Purus Arc and a period of mega-wetland formation in the proto-Orinoco-Amazonas (Lundberg et al. 1998; Albert and Reis, 2011). The uplift of this arc is due to an orogenetic response to the initial elevation of the Andes Mountain range, which consequently may have caused allopatric speciation in some fish species (Armijo et al. 2015). This context of allopatry is reinforced by the presence of fossils of C. macropomum, known as pacus, in the Magdalena River basin in Colombia, which today is an inhabitant of the Amazon and Orinoco rivers. This has suggested that this taxon inhabited ancient systems that connected the Amazon and Magdalena River basins, today separated by the Andes Mountain range (Lundberg et al. 2010). Ecological factors have also been associated with the diversification of frugivorous pacus, which coincides with the diversification of fruit plants during the Eocene (Correa et al. 2015). However, with regard to chromosomal aspects, the subfamily Cossolomatinae was the group that presented descending dysploidy (2n = 54), which has not presented an increase in chromosomal diversification, contrary to the subfamily Serrasalminae, constituted by an ascending to dysploidy with 2n = 58 to 64.
During the Lower Eocene and Upper Miocene (30 − 20 Mya), the uplift of the Andes in the North and Central region culminated in a change in the course of several rivers (Lundberg et al. 1998). These orogenetic processes significantly altered the hydrography in South America, leading to fracturing processes in several basins, with redirection of river courses and headwater capture events (Hoorn et al. 2010; Evenstar et al. 2015). These processes may have provided ecological opportunities and colonization of Serrasalmideos for new habitats, providing speciation events (Melo et al. 2018; Roxo et al. 2019; Ochoa et al. 2020; Melo et al. 2021). Together with these cladogenetic mechanisms, they may have triggered chromosomal rearrangements with a tendency towards ascending dysploidy. However, our biogeographic reconstruction demonstrated that dispersal/ vicariance events apparently did not accompany karyotypic changes.
As the ancestral chromosome reconstruction shows, ascending dysploidy lead to higher chromosome numbers at the most derived lineages of Serrasalminae, culminating in the 2n = 60 karyotype being predominant in the Serrasalmus and Pygocentrus genera. In addition to this, our data revealed an increase in the diversification rate during the Miocene (11 − 8 Mya), involving these genera. These findings are in agreement with other studies, which also pointed to rapid and recent radiation involving this group of piranhas, which have diversified greatly in the plains of South America (Hubert and Renno, 2006; Hubert et al. 2007). The apparent correlation between the 2n = 60 karyotypes and the increase in diversification rate, coupled with the tendency for ascending dysploidy in the family, may point to a scenario in which high chromosome numbers are associated with species diversification and evolutionary success. However, causation would be hard to infer until further and more detailed genomic studies for these groups are provided.
During the late Miocene and Pliocene, hydrological and paleogeographic events may have driven changes in the diversification rates of this group. In relation to other river basins, the Amazon separated from the Paraná-Paraguay system by 10 Ma, leading to the separation of the ichthyofauna in these systems (Hubert and Renno, 2006). Headwater capture events have also been identified between the Upper Paraná and São Francisco basins around 10 Mya (Hubert and Renno, 2006). Dispersions such as separations between watersheds, provided by tectonic activations and changes in the course of rivers (geodispersion), may have provided a certain advantage for carnivorous piranhas, in dispersing species and conquering new habitats, reducing predation and competition. These processes can lead to increased rates of speciation and diversification, as happened in the subfamily Hipostominae (Cardoso et al. 2012). In addition to orogenetic movements, sea level fluctuations may also have further contributed to promoting this diversification (Hubert and Renno 2006). In the last 10 Mya, the sea level has varied from 35 m above to 122 m below the current level and may also have contributed to accelerate this process (Hansen et al. 2013; Hubert et al. 2007). In this context, in addition to allopatric speciation, processes of sympatric speciation have also been detected in some sister lineages of piranhas, associated with habitat heterogeneity. As is the case of S. compressus n = 30 and S. hollandi n = 32, which live in the Madeira River and diverged in the last 2 Ma (Hubert et al. 2007).
All the discussed scenarios highlight that historical and ecological processes seem to have shaped this family's genetic and phylogenetic diversity, involving several types of chromosomal rearrangements. In this context, ascending dysploidy seems to have driven the karyotypic evolution of some lineages towards higher chromosome numbers,resultin in highly diversified persistent lineages across different hydrographic basins of South America. These data, combined with previous studies (Correa et al. 2015), demonstrate that the rate of diversification in Serrasalmidae is not correlated with biogeographic changes, but that it could possibly be linked to ecological processes that lead to morphological changes (Kolmann et al. 2021), as well as karyoevolutionary differentiation by dysploidy. Generally, dysploidy does not necessarily imply changes in DNA content, only in the structure of chromosomal rearrangements that can occur in the genome. These processes, so far, have been considered to have a neutral effect in relation to the diversification of evolutionary processes over the long term (Escudero et al. 2014).