Investigating Diadromy in Fishes and Its Loss in an -Omics Era

Abstract

Summary Diadromy, the predictable movements of individuals between marine and freshwater environments, is biogeographically and phylogenetically widespread across fishes. Thus, despite the high energetic and potential fitness costs involved in moving between distinct environments, diadromy appears to be an effective life history strategy. Yet, the origin and molecular mechanisms that underpin this migratory behavior are not fully understood. In this review, we aim first to summarize what is known about diadromy in fishes; this includes the phylogenetic relationship among diadromous species, a description of the main hypotheses regarding its origin, and a discussion of the presence of non-migratory populations within diadromous species. Second, we discuss how recent research based on -omics approaches (chiefly genomics, transcriptomics, and epigenomics) is beginning to provide answers to questions on the genetic bases and origin(s) of diadromy. Finally, we suggest future directions for -omics research that can help tackle questions on the evolution of diadromy.

Genomic basis of the loss of diadromy inGalaxias maculatus: Insights from reciprocal transplant experiments

“…In some cases, the transition to residency is abrupt and man‐made imposed by the construction of physical barriers such as river dams that landlock populations; but in others, resident populations exist even though they have access to the sea. The presence of facultative diadromous species or diadromous and resident populations inhabiting the same locations (sympatry) suggest that the decision to remain in their natal environment can be due to ecological factors, such as lenient environmental conditions and intra‐specific competition, that reduced migratory costs and increased survival (Delgado & Ruzzante, 2020).…”

An investigation of genetic connectivity shines a light on the relative roles of isolation by distance and oceanic currents in three diadromous fish species

“…Common galaxias G. maculatus is a widespread species with a distribution extending across Australia, New Zealand and South America (Gomon and Bray 2011). The form of diadromy exhibited by common galaxias is still debated, but is considered either amphidromous or semicatadromous, with adults migrating into estuaries to breed and larvae drifting into the marine environment before immigrating into rivers as juveniles (McDowall 2000;Bice et al 2019b;Delgado and Ruzzante 2020). Adults produce thousands of eggs that hatch and undertake an ,5 month marine larval phase (McDowall et al 1994).…”

“…Many life history traits have been shown to influence connectivity, with higher connectivity typically seen in diadromous species that show higher fecundities, larger body size or long-lasting pelagic larval stages, or those that actively disperse (Selkoe and Toonen 2011;Clobert et al 2012;Feutry et al 2013;Jones and Closs 2016). Although some economically important diadromous species such as salmonids have been studied extensively, diadromous species as a whole are a critically understudied group of fish, with many key questions surrounding how diadromy affects the overall connectivity seen within different diadromous species remaining (McDowall 1999;Delgado and Ruzzante 2020). This has ultimately resulted in insufficient knowledge of life history for many species to estimate dispersal.…”

Contrasting Patterns of Genetic Diversity and Divergence Between Landlocked and Migratory Populations of Fish Galaxias maculatus, Evaluated Through Mitochondrial DNA Sequencing and Nuclear DNA Microsatellites

“…In addition, the presence of resident or landlocked populations has been observed in lakes and rivers, as well as migratory populations that maintain amphidromous behaviour ( Delgado et al, 2019 ). This species is therefore considered an interesting model for historical biogeographic studies due to its particular distribution ( Waters and Burridge, 1999 ; Waters et al, 2000 ; Zattara et al, 2005 ; Burridge et al, 2012 ; Waters et al, 2020 ), its response to glacial events ( Zemlak et al, 2010 ; Zemlak et al, 2011 ; Carrea et al, 2012 , 2013 ; González-Wevar et al, 2015a ; González-Wevar et al, 2015b ; Victoriano et al, 2020 ), and its migratory and non-migratory behaviour in the same habitat ( Delgado et al, 2019 ; Delgado and Ruzzante, 2020 ). This variability makes G. maculatus a suitable model for evaluating genetic indicators associated with these unique characteristics, as this species has been considered to represent an intermediate evolutionary step between migratory behaviours of marine and freshwater organisms ( Corush, 2019 ).…”