Biogeographic context and sampling design around the island of Crete in Mediterranean Gouania.

(a) Distribution ranges and sampling sites of the five Gouania species, shown in different colors. Sympatric species pairs are indicated by half-filled circles. The background map is colored according to a Mediterranean biogeographic model comprising seven major ecoregions (sensu Spalding et al., 2007) and includes the major surface circulation patterns (El-Geziry & Bryden, 2010). Species illustrations are shown above their respective distribution ranges. (b) Sampling localities and (c) studying design around the island of Crete and Kythira. The study region is located at the intersection of three Mediterranean ecoregions (Ionian, Aegean and Levantine Seas) and therefore provides an ideal natural setting to investigate how shared oceanographic conditions influence connectivity and genomic divergence. Comparative whole-genome analyses focused on the two sympatric eastern Mediterranean species, G. orientalis and G. hofrichteri.

Passive larval drift simulation as a null model for dispersal abilities in Gouania.

(a) The five investigated populations are embedded in a complex seascape with three main gyres and jets: the Pelops Gyre (PG), the Western Cretan Gyre (WCG), the Ierapetra Gyre (IG), the Mid-Ionian Jet (MIJ) and the Mid-Mediterranean Jet (MMJ). Arrows represent mean currents in 0.5° x 0.5° grids This map was adapted from published drift data (Poulin et al., 2013). (b–e) Lagrangian simulations of passive larval drift around Crete and Kythira across four seasons: (b) winter (November–March), (c) spring (March–June), (d) summer (June–September) and (e) autumn (September–November). Particles were released daily from the four sampled populations (Kythira, NW-Petres, NE-Mades and southern Crete) and tracked for 13 days. Each point represents particle positions at 6-hour intervals.

Parallel population genomic structure across sympatric Gouania species around the island of Crete.

Principal components analysis of biallelic SNPs (MAF=0.1) for G. orientalis (a) and G. hofrichteri (b). Colored dots correspond to sampling locations shown on the map above. Roman numerals (I–III) indicate regions of increased network complexity. (c) Pairwise genome-wide mean FST between populations for G. orientalis (lower diagonal) and G. hofrichteri (upper diagonal). (d) Distribution of genome-wide inbreeding coefficients (FIS) across populations for both species. Elevated FIS values in Kythira are consistent across species, suggesting increased inbreeding and/or stronger population subdivision relative to the Cretan populations.

Population connectivity among Gouania orientalis and G. hofrichteri populations around the island of Crete and Kythira.

D- and f4-statistics were calculated with Dsuite (P1-P3 sorted by phylogeny). p-values were adjusted for multiple testing applying a Benjamini-Hochberg correction and significant trios are presented in bold. For the G. orientalis dataset, we excluded the single population sample from NE-Mades as it led to biases in the statistics.

Parallel and species-specific genomic signals between sympatric Gouania orientalis and G. hofrichteri Kythira and Crete populations.

Maximum PBS values for the Kythira population, summarized across all trio comparisons, are shown for (a) G. orientalis and (b) G. hofrichteri. Orange-outlined points indicate genomic windows identified as outliers by both dA and PBS, while red circles highlight parallel outlier windows shared between the two species. Arrows indicate genes found within the surrounding area of outlier windows (a full list is shown in Table S6). Furthermore, density distributions of ΔTajima’s D (Tajima’s DKythira − mean Tajima’s DCrete) compare candidate windows against the genomic background, revealing no significant shift towards more negative values in (c) G. orientalis, but a significant enrichment of more negative values in (d) G. hofrichteri.

Bio-and phylogeographic patterns based on COI data of all five closely related beach fishes of the genus Gouania.

(a-e) TCS haplotype networks are based on partial COI sequences for each of the five Gouania species. For each network two circles representing the size of ten and one sample are given. The numbers before each location name in brackets refers to the same numbers in grey circles corresponding to sampling sites shown in the centered map. Note that because of their vicinity, populations from Canges-sur-mer and Antibes show the same number in the map. For a detailed list of samples per population see Table S2.

Demographic histories inferred using MSMC2 for all five Gouania species.

(a) Post-Messinian radiation of the genus Gouania. Phylogenetic relationships among species are shown based on a simplified dated phylogeny inferred from nine nuclear and one mitochondrial marker (Wagner et al., 2019). (b) G. orientalis (Saronida, Attica, Greece), G. hofrichteri (Chamolia, Attica, Greece), G. adriatica (Pula, Croatia), G. pigra (Pula, Croatia) and G. willdenowi (Toulon, France). For each species, 20 bootstrap replicates are shown as dashed lines, with the main estimate represented by a solid line. Grey shading indicates the last glacial maximum (LGM) at around 20,000 years BP.