3.2 Population structure and sex-biased dispersal
Mitochondrial and nuclear results from Fu’s Fs indicated no deviation
from neutrality (Supplementary Material 7). However, for Tajima’s D
tests, three localities displayed significant negative Tajima’s D for
all mitochondrial and nuclear markers: South Reunion (Indian Ocean, see
map in Figure 1), Saint-Barthélemy (W Atlantic) and Raso (E Atlantic).
In addition, three localities presented significant negative Tajima’s D
at mitochondrial loci only (Selvagem and Funchal, E Atlantic; South
Reunion) and two at nuclear loci only (Vila, E Atlantic; North Reunion).
Patterns of population structure at seven out of 13 localities might
therefore be influenced by selection and/or recent demographic changes,
in addition to neutral processes.
Gene trees and phylogenetic inference with *BEAST and MrBayes revealed a
hierarchical structure composed of two well-supported (posterior
probabilities PP ≥ 0.95) reciprocally monophyletic clades corresponding
to the two oceans, within which individuals from the five lineages
further clustered into monophyletic sub-clades (Fig. 2b,c and S7). All
except one of these sub-clades (East Atlantic boydi ) were
supported in *BEAST (PP ≥ 0.95) using all concatenated markers. For both
mtDNA markers, and all markers concatenated, the central-Pacificdichrous lineage was nested within thebailloni /nicolae clade, although node supports fordichrous position within Indian Ocean clade were weak (between
0.27 and 0.73). Assignment to an ocean basin based on nuclear haplotype
networks was however discordant from the mitochondrial data for 33
individuals, for at least one nuclear locus (Fig. 3b and S8): 15
Atlantic individuals fell closely to the Indian phylogroup, and 18
Indian Ocean individuals clustered within the Atlantic group. All of
these 33 individuals showed the mitochondrial signature expected based
on their geographical sampling location. Interestingly, a baroliindividual showed one haplotypic phase clustering with the baroliphylogroup (mother) while the other haplotypic phase (father) clustered
with the nicolae phylogroup for four nuclear markers (the two
remaining could not be assigned to any particular lineages). This
individual might be the result of hybridization, although further
analyses based on additional markers would be necessary to detect more
robustly hybridization among lineages. The ambiguous assignment of the
other individuals might be due to introgression or incomplete lineage
sorting (see below).
In parallel, we used an AMOVA framework with the five nominal lineages
now defined a priori, to examine how genetic variants partitioned among
and within these taxonomic units. Most of the genetic variance was due
to inter-lineage differentiation (88.5% and 58.4% for mitochondrial
and nuclear markers, respectively). The variance among sampling
localities within lineages accounted for 0.5 to 4.1%, while variance
within sampling localities represented 11.0 to 37.5%. PairwiseF ST showed consistently higher values among, than
within lineages for both marker types, with mostly non-significant
values within each lineage (Table 2a). Moreover, 24 nuclear
FST values were found non-significant versus 10
mitochondrial ΦST values (Table 2a). Population average
pairwise differences led to similar results, with high structuration for
the five nominal lineages (Supplementary Material 7).
Genetic distance increased clearly with geographic distance (Fig. 4a),
but Mantel tests were performed only between pairs within the same Ocean
(given that each Ocean taxon is likely different species). Tests
confirmed that genetic and geographic distances were strongly correlated
to each other, both for mtDNA and nuclear markers when analyzing pairs
of populations within an Ocean basin (r=0.88 and 0.70, n=45, p
<0.005 for mtDNA and nuDNA, respectively, Fig. 4a,b). Between
breeding sites and within lineages, isolation by distance could not be
reliably tested as the number of populations was too low, but visually
it seemed that there was no relation between geographic and genetic
distances (Fig. 4).
Indian Ocean populations (nicolae and bailloni ) showed
stronger female dispersion as indicated by significantly stronger
deficit of heterozygotes and a significantly lower average relatedness
in females (Table 3). Conversely, in baroli ,F IS was significantly higher for males and they
were less related to each other than females, suggestive of male-biased
dispersal. Finally, an ambiguous pattern was found for bothlherminieri (males had stronger deficit of heterozygotes and were
significantly more related to each other than females) and boydi(female F IS was significantly higher than maleF IS, though females were more related to each
other than for males at one sampling locality). In addition, population
structure within lineages, as measured with F ST,
was similar between sexes, but between oceans a larger range ofF ST values was observed for males with higher
maximum values, suggesting that males were more structured at least for
some pairs of populations (e.g. lherminieri vs. Indian Ocean
lineages; Table 2b). Overall males seemed more structured than females
between oceans, suggesting that females disperse farther, but genetic
signal for sex-biased dispersal varied geographically: female-biased in
the Indian Ocean, male-biased or inconclusive in the Atlantic Ocean.