Host-parasite interactions in perpetual darkness: Macroparasite diversity in the cavefish Astyanax mexicanus

Astyanax mexicanus has repeatedly colonized cave environments, displaying evolutionary parallelisms in many troglobitic traits. Despite being a model system for the study of adaptation to life in perpetual darkness, the parasites that infect cavefish are practically unknown. In this study, we investigated the macroparasite communities in 18 cavefish populations from independent lineages and compared them with the parasite diversity found in their sister surface fish populations, with the aim of better understanding the role that parasites play in the colonization of new environments. Within the cavefish populations, we identified 13 parasite taxa, including a subset of 10 of the 27 parasite taxa known for the surface populations. Parasites infecting the cavefish belong to five taxonomic groups, including trematodes, monogeneans, nematodes, copepods, and acari. Monogeneans are the most dominant group, found in 14 caves. The macroparasites include species with direct life cycles and trophic transmission, including invasive species. Surprisingly, paired comparisons indicate higher parasite richness in the cavefish than in the surface fish. Spatial variation in parasite composition across the caves suggests historical and geographical contingencies in the host-parasite colonization process and potential evolution of local adaptations. This base-line data on parasite diversity in cavefish populations of A. mexicanus provides a foundation to explore the role of divergent parasite infections under contrasting ecological pressures (cave vs. surface environments) in the evolution of cave adaptive traits.

Mitochondrial phylogeography and molecular evolution of the rhodopsin visual pigment in troglobitic populations of Astyanax mexicanus (De Filippi, 1853)

Cave-adapted animals provide a unique opportunity to study the evolutionary mechanisms underlying phenotypic, metabolic, behavioral, and genetic evolution in response to cave environments. The Mexican tetra ( Astyanax mexicanus) is considered a unique model system as it shows both surface and cave-dwelling morphs. To date, at least 33 different cave populations have been identified, with phylogenetic studies suggesting an origin from at least two independent surface lineages, thereby providing a unique opportunity to study parallel evolution. In the present study, we carried out the most exhaustive phylogeographic study of A. mexicanus to date, including cave and surface localities, using two mitochondrial markers (cytochrome b (cyt b) and cytochrome c oxidase subunit I ( COI)) and nuclear rhodopsin visual pigment ( rho). Additionally, we inferred the molecular evolution of rho within the two contrasting environments (cave and surface) and across three geographic regions (Sierra de El Abra, Sierra de Guatemala, and Micos). In total, 267 individuals were sequenced for the two mitochondrial fragments and 268 individuals were sequenced for the rho visual pigment from 22 cave and 46 surface populations. Phylogeographic results based on the mitochondrial data supported the two-lineage hypothesis, except for the Pachón and Chica caves, whose introgression has been largely documented. The Sierra de El Abra region depicted the largest genetic diversity, followed by the Sierra de Guatemala region. Regarding the phylogeographic patterns of rho, we recovered exclusive haplogroups for the Sierra de El Abra (Haplogroup I) and Sierra de Guatemala regions (Haplogroup IV). Moreover, a 544 bp deletion in the rho gene was observed in the Escondido cave population from Sierra de Guatemala, reducing the protein from seven to three intramembrane domains. This change may produce a loss-of-function (LOF) but requires further investigation. Regarding nonsynonymous ( dN) and synonymous ( dS) substitution rates (omega values ω), our results revealed the prevailing influence of purifying selection upon the rho pigment for both cave and surface populations (ω<1), but relaxation at the El Abra region. Notably, in contrast to the other two regions, we observed an increase in the number of dN mutations for Sierra de El Abra. However, given that a LOF was exclusively identified in the Sierra de Guatemala region, we cannot dismiss the possibility of a pleiotropic effect on the Rho protein.