Patterns of morphological variation among populations of the widespread annual killifish Nothobranchius orthonotus are independent of genetic divergence and biogeography

Genetic diversity of a widespread annual killifish from coastal Tanzania

Background

African annual killifishes (Nothobranchius spp.) are adapted to seasonally desiccating habitats (ephemeral pools), surviving dry periods as dormant eggs. Given their peculiar life history, geographic aspects of their diversity uniquely combine patterns typical for freshwater taxa (river basin structure and elevation gradient) and terrestrial animals (rivers acting as major dispersal barriers). However, our current knowledge on fine-scale inter-specific and intra-specific genetic diversity of African annual fish is limited to a single, particularly dry region of their distribution (subtropical Mozambique). Using a widespread annual killifish from coastal Tanzania and Kenya, we tested whether the same pattern of genetic divergence pertains to a wet equatorial region in the centre of Nothobranchius distribution.

Results

In populations of Nothobranchius melanospilus species group across its range, we genotyped a part of mitochondrial cytochrome oxidase subunit 1 (COI) gene (83 individuals from 22 populations) and 10 nuclear microsatellite markers (251 individuals from 16 populations). We found five lineages with a clear phylogeographic structure but frequent secondary contact. Mitochondrial lineages were largely congruent with main population genetic clusters identified on microsatellite markers. In the upper Wami basin, populations are isolated as a putative Nothobranchius prognathus, but include also a population from a periphery of the middle Ruvu basin. Other four lineages (including putative Nothobranchius kwalensis) coexisted in secondary contact zones, but possessed clear spatial pattern. Main river channels did not form apparent barriers to dispersal. The most widespread lineage had strong signal of recent population expansion.

Conclusions

We conclude that dispersal of a Nothobranchius species from a wet part of the genus distribution (tropical lowland) is not constrained by main river channels and closely related lineages frequently coexist in secondary contact zones. We also demonstrate contemporary connection between the Ruvu and Rufiji river basins. Our data do not provide genetic support for existence of recently described cryptic species from N. melanospilus complex, but cannot resolve this issue.

Plio-Pleistocene phylogeography of the Southeast Asian Blue Panchax killifish, Aplocheilus panchax

The complex climatic and geological history of Southeast Asia has shaped this region’s high biodiversity. In particular, sea level fluctuations associated with repeated glacial cycles during the Pleistocene both facilitated, and limited, connectivity between populations. In this study, we used data from two mitochondrial and three anonymous nuclear markers to determine whether a fresh/brackish water killifish, Aplocheilus panchax, Hamilton, 1822, could be used to further understand how climatic oscillations and associated sea level fluctuations have shaped the distribution of biota within this region, and whether such patterns show evidence of isolation within palaeodrainage basins. Our analyses revealed three major mitochondrial clades within A. panchax. The basal divergence of A. panchax mitochondrial lineages was approximately 3.5 Ma, whilst the subsequent divergence timings of these clades occurred early Pleistocene (~2.6 Ma), proceeding through the Pleistocene. Continuous phylogeographic analysis showed a clear west-east dispersal followed by rapid radiation across Southeast Asia. Individuals from Krabi, just north of the Isthmus of Kra, were more closely related to the Indian lineages, providing further evidence for a freshwater faunal disjunction at the Isthmus of Kra biogeographic barrier. Our results suggest that Sulawesi, across the Wallace Line, was colonised relatively recently (~30 ka). Nuclear DNA is less geographically structured, although Mantel tests indicated that nuclear genetic distances were correlated with geographic proximity. Overall, these results imply that recent gene flow, as opposed to historical isolation, has been the key factor determining patterns of nuclear genetic variation in A. panchax, however, some evidence of historical isolation is retained within the mitochondrial genome. Our study further validates the existence of a major biogeographic boundary at the Kra Isthmus, and also demonstrates the use of widely distributed fresh/brackishwater species in phylogeographic studies, and their ability to disperse across major marine barriers in relatively recent time periods.

Community assembly in Nothobranchius annual fishes: Nested patterns, environmental niche and biogeographic history

The assembly of local communities from regional species pools is shaped by historical aspects of distribution, environmental conditions, and biotic interactions. We studied local community assembly patterns in African annual killifishes of the genus Nothobranchius (Cyprinodontiformes), investigating data from 168 communities across the entire range of regionally co-existing species. Nothobranchius are small fishes associated with annually desiccating pools. We detected a nested pattern of local communities in one region (Southern Mozambique, with Nothobranchius furzeri as the core and dominant species), but no nestedness was found in the second region (Central Mozambique, with Nothobranchius orthonotus being the dominant species). A checkerboard pattern of local Nothobranchius community assembly was demonstrated in both regions. Multivariate environmental niche modeling revealed moderate differences in environmental niche occupancy between three monophyletic clades that largely co-occurred geographically and greater differences between strictly allopatric species within the clades. Most variation among species was observed along an altitudinal gradient; N. furzeri and Nothobranchius kadleci were absent from coastal plains, Nothobranchius pienaari, Nothobranchius rachovii, and Nothobranchius krysanovi were associated with lower altitude and N. orthonotus was intermediate and geographically most widespread species. We discuss implications for ecological and evolutionary research in this taxon.

Female fecundity traits in wild populations of African annual fish: the role of the aridity gradient

The evolution of life history is shaped by life expectancy. Life-history traits coevolve, and optimal states for particular traits are constrained by trade-offs with other life-history traits. Life histories contrast among species, but may also diverge intraspecifically, at the level of populations. We studied the evolution of female reproductive allocation strategy, using natural populations of two sympatric species of African annual fishes, Nothobranchius furzeri and Nothobranchius orthonotus. These species inhabit pools in the Mozambican savanna that are formed in the rainy season and persist for only 2-10 months. Using 207 female N. furzeri from 11 populations and 243 female N. orthonotus from 14 populations, we tested the effects of genetic background (intraspecific lineage) and life expectancy (position on the aridity gradient determining maximum duration of their temporary habitat) on female fecundity traits. First, we found that variation in female body mass was small within populations, but varied considerably among populations. Second, we found that fecundity was largely defined by female body mass and that females spawned most of their eggs in the morning. Third, we found that the trade-off between egg size and egg number varied among lineages of N. furzeri and this outcome has been confirmed by data from two separate years. Overall, we demonstrate that local conditions were important determinants for Nothobranchius growth and fecundity and that eggs size in arid region was less limited by female fecundity than in humid region.