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)