Correlations with ocean salinity
To investigate the potential roles of ploidy and subgenome composition
in adaptation to salt tolerance, we compared the surface salinity of the
ocean nearest to the collection location of each P. vaginatumsample. Salinity levels did not differ significantly between the
locations of diploid and triploid individuals (P = 0.5328, Fig. 5);
thus, ploidy variation alone does not obviously contribute to adaptive
variation in salinity tolerance. In contrast, a linear model that
incorporated both the number of P. vaginatum subgenomes (one vs.
two, independent of ploidy) and the number of the unidentified
subgenomes (zero, one, or two) revealed that having two P.
vaginatum subgenomes was strongly associated with higher local ocean
salinity (P = 7.6e-11 ), while having an additional copy of the unknown
genome was weakly associated with decreased salinity (P = 0.033, Fig.
5). This finding suggests that the relative proportion of subgenomes
from the halophytic species has a detectable effect on salinity
adaptation. It should be noted, however, that we could not entirely
remove the effect of ploidy from this analysis because after accounting
for the number of P. vaginatum subgenomes, a change in the copy
number of the other subgenome necessarily changes ploidy as well (2:0 to
2:1 and 1:1 to 1:2).