Thorichthys panchovillai, Del Moral-Flores, López-Segovia & Hernández-Arellano, 2017

ELÍAS, DIEGO J., BETANCOURT-RESENDES, ISAÍ, DÍAZ-FLORES, ALEJANDRO, CAMAK, DAVID T., DOMÍNGUEZ-DOMÍNGUEZ, OMAR, ARTIGAS-AZAS, JUAN M., PILLER, KYLE R. & MCMAHAN, CALEB D., 2025, Evolutionary history of the polymorphic Coatzacoalcos River endemic Thorichthys panchovillai (Cichliformes: Cichlidae), Zootaxa 5618 (1), pp. 29-46 : 39-41

Phylogenetic placement of Thorichthys panchovillai

Thorichthys panchovillai is the most recently described species of Thorichthys ( Del-Moral-Flores et al. 2017) and until now no study has investigated its phylogenetic placement using molecular data. Our phylogenetic hypothesis recovered T. panchovillai nested within a clade comprised of T. aureus , T. callolepis , T. helleri , T. maculipinnis , and T. socolofi ( Fig. 2 ). This clade is largely distributed in the Atlantic versant of the northern Neotropics from the Motagua River in Honduras and Guatemala to the Papaloapan River in Mexico. The majority of species in this clade are allopatrically distributed, with the exception of T. panchovillai and T. callolepis which occur syntopically in the upper reaches of the Coatzacoalcos basin ( Fig. 4 ).

Our results recovered all of our samples of T. panchovillai and two samples archived in GenBank ( T. aff maculipinnis KU 854667-68; see Table 1 ) from the Coatzacoalcos basin as monophyletic ( Fig. 2 ). These two sequences were deposited in GenBank in the year 2016, prior the description of T. panchovillai ( Del-Moral-Flores et al. 2017). Therefore, we treat all samples that belong to this clade as T. panchovillai . Our results recovered T. panchovillai sister to T. socolofi with strong support (BPP =1, BS = 92; Fig. 2 ). Thorichthys socolofi is a narrowly distributed species, found in the Tulijá River, which is a tributary of the lower Grijalva basin located east of the Coatzacoalcos River. Our results are in contrast with the evolutionary hypothesis of relationships proposed by Del-Moral-Flores et al. (2017). Based on similarity of meristic characters (i.e., gill rakers, dorsal and anal spines, and fin ray counts), the authors hypothesized that T. panchovillai was sister to T. maculipinnis , a species widely distributed in the Papaloapan River, the adjacent river basin west of the Coatzacoalcos River. It is worth noting that the observed genetic divergence between T. panchovillai and T. socolofi (K2P = 1.98%, S.E. = 0.44) is the lowest genetic divergence for any species pair comparison between species of Thorichthys ( Fig. 3 , Table 2 ), suggesting that these lineages may have diverged recently in the evolutionary history of Thorichthys . In contrast, the observed genetic divergence between T. panchovillai and T. maculipinnis was relatively high (K2P = 7.28 5, S.E. = 0.86), suggesting an older time of divergence supporting our phylogenetic hypothesis.

Although described from the upper Coatzacoalcos River, Thorichthys panchovillai is widely distributed in the basin ( Del-Moral-Flores et al. 2017, Artigas Azas 2017). Our results uncovered low genetic diversity and we recovered four haplotypes across its distribution, two of which are common and two that are unique ( Fig. 4B ). Interestingly, two haplotypes are restricted to our samples from the upper reaches of the Coatzacoalcos and two haplotypes are restricted to the lower reaches of the Coatzacoalcos basin ( Fig. 4B ). The presence of unique mitochondrial haplotypes in the upper reaches of river basins in the northern Neotropics has been reported in other freshwater fishes ( Elías et al. 2020, 2022, Rocamontes-Morales et al. 2021) and it is likely that these patterns are the result of the complexity of river networks ( Thomaz et al. 2016), paleoclimatic events (e.g., Elías et al. 2020, Rocamontes-Morales et al. 2021) or a combination of both. One caveat of our study is that we do not have a continuous sampling across the Coatzacoalcos riverscape and alternatively the recovered pattern can be the result of isolation by distance. Future work that includes a more robust sampling along the Coatzacoalcos will shed light on the spatial distribution of the genetic diversity of T. panchovillai .

Interestingly, the spatially segregated haplotypes along the Coatzacoalcos riverscape can be matched to alternative morphotypes sensu Artigas Azas (2017), the blue mixteco in the upper reaches and the gold mixteco in the lower reaches (see Fig. 4B ). The spatial distribution of the morphotypes of T. panchovillai ( Fig. 5 ) seems to be spatially segregated ( Artigas Azas 2017). The gold mixteco form has been reported from the lower reaches of the Coatzacoalcos and Jaltepec sub-basins (see Figs. 1B and 5D ). The green mixteco appears to be restricted to the Uxpanapa sub-basin ( Figs. 1B and 5C ) and the blue mixteco is restricted to the upper reaches of the Coatzacoalcos, Tolosa, and Chachijapa, sub-basins ( Figs. 1B , 4B , and 5A, B ) ( Artigas Azas 2017).

It is unclear what factors are driving this polymorphism within T. panchovillai . The blue mixteco morphotype presents a more conspicuous coloration pattern in contrast with the other two morphotypes (see Figs. 4B and 5 ) and a plausible explanation is that sexual selection might be driving this pattern. The blue mixteco morphotype of T. panchovillai is syntopic with T. callolepis ( Miller & Nelson 1961, Del-Moral-Flores et al. 2017; author field observations; Fig. 5 ). The observed genetic divergence between T. panchovillai and T. callolepis (K2P = 8.34, S.E. = 0.94), is one of the largest genetic divergences observed within species pairs in this clade ( Figs. 2 and 3 , Table 2 ). These results lead us to hypothesize that these syntopic populations of T. panchovillai and T. callolepis are potentially the result of secondary contact after a long time of geographic isolation, and that the blue mixteco morphotype could be the result of assortative mating, in which the co-existence with a congeneric ( T. callolepis ; Fig. 5 ) led to selective pressure in coloration patterns reinforcing the species boundaries in Thorichthys species via visual cues ( Elmer et al. 2009, Maan & Sefc 2013, Rometsch et al. 2020). Color-assortative mating has been proposed as a mechanism that helps to maintain species boundaries of species that arise in allopatry and experience secondary contact ( Salzburger et al. 2005). Future work that incorporates comprehensive geographic sampling across the Coatzacoalcos basin, coupled with genomic scale data and ex situ interspecific mating experiments will help to test our proposed hypotheses. Overall, these results demonstrate that the genus Thorichthys is a novel study system to explore the drivers of speciation in northern Neotropical riverine cichlids.

Figures

FIGURE 1. (A)Geographic locationof the Coatzacoalcos watershed and its adjacent river basins intheAtlantic slope of Southern Mexico. (B) River sub-basins within the Coatzacoalcos watershed, 1) Coatzacoalcos sub-basin [pale grey], 2) Jaltepec sub-basin [mint], 3) Tolosa sub-basin [pink], 4) Chachijapa sub-basin [olive], and 5) Uxpanapa sub-basin [orange]. River sub-basin names follows Instituto Nacional de Estadística y Geografía (INEGI) Mexico. (https://www.inegi.org.mx/temas/hidrografia/). Dashed line delineates political boundaries at the state level within Mexico.

FIGURE 2. (A) Phylogenetic relationships of Thorichthys, topology inferred under bayesian approach. Support values for the nodes represent Bayesian Posterior Probabilities [BPP] and bootstrap support values [BS]; BPP / BS -. Asterisk represents samples of T. affinis from the type locality, Lake Petén Itzá, Guatemala. (B) Geographic locality of samples/sequences of T. affinis [red triangle], T. cf. helleri [yellow square] T. meeki [red square], and T. pasionis [grey square]. Hydrographic regions: Papaloapan basin [1], Coatzacoalcos basin [2], and Tonala basin [3]. (C) Geographic locality of samples/sequences of T. aureus [blue circle], T. callolepis [teal circle], T. helleri [yellow circle] T. maculipinnis [orange circle], T. panchovillai [purple circle], and T. socolofi [green circle]. For locality information see Table 1. Hydrographic regions: Grijalva-Usumacinta [4] and Polochic- Cahabón [5] areas of endemism sensu Elías et al. 2020, and Motagua basin [6].

FIGURE 3. Estimated genetic divergence using the p-distance (grey triangles) and the Kimura 2 parameter model (black circles) and estimated standard error based on the only-Thorichthys alignment. Comparison between the two major clades of Thorichthys inferred with the mitochondrial Cytb marker and between T. panchovillai (n=12) and closely related species: T. callolepis (n =4), T. maculipinnis (n =3), T. helleri (n =3), T. socolofi (n =4), and T. aureus (n =3).

FIGURE 4. (A) Map showing geographic localities of our sampling of Thorichthys panchovillai across the Coatzacoalcos riverscape.Area of sympatry between T. panchovillai and T. callolepis denoted by dashed region in the Tolosa [pink], Chachijapa [olive], and upper Coatzacoalcos [pale grey] sub-basins. (B) Phylogenetic placement T. panchovillai and haplotype network based on analysis of 15 sequences across the Coatzacoalcos River. Numbers inside the circles represent the number of sequences that match that unique haplotype. Photographs depicting the morphotypes observed across the Coatzacoalcos riverscape. Black dots in the area of sympatry between T. panchovillai and T. callolepis reflect localities from the populations that were used to describe T. panchovillai: georeference data taken from Del-Moral-Flores et al. (2017).

FIGURE 5. Live photographs of Thorichthys panchovillai morphotypes (A) female of the blue mixteco morphotype in reproductive coloration from the Ajal River in the Tolosa sub-basin, (B) male with fry of the blue mixteco morphotype from the Chalchijapan River in the Chachijapa sub-basin, (C) female of the green mixteco morphotype from the headwaters of the Uxpanapa sub-basin, (D) adult individual of the gold mixteco morphotype from the Jaltepec sub-basin. And (E) Thorichthys callolepis a species of Thorichthys that occurs in sympatry with the blue mixteco form of T. panchovillai in the upper reaches of the Coatzacoalcos basin, and (D)Thorichthys socolofi the sister species of T. panchovillai. Photographs by Juan M. Artigas- Azas.