3.3.7 Introgression
All NA blue whale samples showed significant introgression (Z-score
>3) with fin whales (Table 2). Four of six historical blue
whale samples also showed evidence of introgression with the fin whale
genome. The late twentieth century sample, NWa-CM1 (1974), had a high
D-statistic value (0.94), indicating that it originated from a recent
hybridization between a fin and blue whale. The mitochondria of this
whale indicated that the hybrid had a blue whale mother. Two other
early-1900s bone samples (NWa5 and NWa6) also had high D-statistics
values, 0.54 and 0.64 respectively, which indicate hybridization in the
more distant past. Blue whales from NA and the humpback whale failed to
show significant interspecific introgression. Likewise, all NA blue
whales, except for NE-Ar (an Icelandic sample sequenced in an earlier
study (Árnason et al., 2018)), failed to show introgression with the sei
whale.
Unidirectional gene flow from fin whale to blue whale was detected via
Dfoil statistics in all present-day blue whale samples (Supplementary
Information, Table S1). The gene flow from fin whales accounted for
~3.5 % of the present-day blue whale genome.
Table 2. D-statistics analysis to detect presence of gene flow
between the blue and fin whales with the four-taxon phylogeny
(((Antarctic Blue, NA Blue), Fin), Minke)