Insights on common dolphin (Delphinus delphis) social organization from genetic analysis of a mass-stranded pod

Abstract

Compared to terrestrial mammals, little is known of cetacean social systems as they are generally less accessible to behavioral investigations due to their aquatic environment. The present study investigates group structure of the pelagic common dolphin, Delphinus delphis, using genetic markers. Tissue samples from 52 individuals representing a recent live mass-stranding event were compared to 42 single strandings taken from presumably different groups. The mass-stranding event occurred in 2002 on the French coast of the English Channel, whereas the single strandings were collected between 1993 and 2003 along the western coast of France (Bay of Biscay and English Channel). Analysis of mitochondrial DNA control region sequences indicated that genetic variability within the mass-stranded pod was similar to variability observed in single strandings. The mass-stranded group was composed of 41 different mitochondrial haplotypes or matrilines while the single strandings revealed 29 different haplotypes. Analysis of 11 microsatellite loci revealed that average relatedness of the mass-stranded pod was not different from average relatedness among all single strandings suggesting that individuals within the group had no closer kin relationships than animals taken from presumably different groups. These results do not support a matriarchal system and suggest that common dolphins constituting a pod are not necessarily genetically related.

Appendix

Microsatellite PCR cycling profiles (see Table 2 for annealing temperature (T _a))

• Ttr04, Ttr11, and Ttr19: 94°C for 30 s, and then 30 cycles of 20 s at 94°C, 20 s at T _a and 30 s at 72°C, and a 10-min final extension at 72°C

• Ttr48 and Ttr34: 94°C for 30 s, and then 28 cycles of 20 s at 94°C, 20 s at T _a, and 20 s at 72°C, and a 10-min final extension at 72°C

• EV14, EV94, and KWM12: 94°C for 30 s, and then 30 cycles of 30 s at 94°C, 30 s at T _a, and 30 s at 72°C, and a 10-min final extension at 72°C

• Ttr63, EV37, and TexVet5: 94°C for 30 s, and then 30 cycles of 30 s at 94°C, 40 s at T _a, and 40 s at 72°C, and a 15-min final extension at 72°C

• Ttr58: 94°C for 30 s, then 10 cycles of 30 s at 94°C, 70°C with a 1°C decrease per step for 40 s, and 40 s at 72°C, and then 20 cycles of 30 s at 94°C, 40 s at T _a, and 40 s at 72°C, and a 15-min final extension at 72°C

• MK5, MK6, MK8, and MK9: 94°C for 3 min, then 10 cycles of 30 s at 92°C, 60°C with a 1°C decrease per step for 30 s, and 1 min at 72°C, and then 24 cycles of 30 s at 92°C, 30 s at T _a, and 1 min at 72°C, and a 10-min final extension at 72°C

• TexVet7: 96°C for 2 min, and then 35 cycles of 30 s at 94°C, 30 s at T _a, and 1 min at 72°C, and a 10-min final extension at 74°C

• PPHO110: 95°C for 30 s, and then 27 cycles of 30 s at 94°C, 30 s at T _a, and 30 s at 72°C, and a 10-min final extension at 72°C

Problems associated with six loci not genotyped in this study

MK9, TexVet5, and PPHO110 did not produce any products, whereas MK6 amplified two non-specific additional fragments in some of the samples. Natoli et al. (2006) amplified TexVet5 successfully in samples from the genus Delphinus and it is noteworthy that amplification of each locus was attempted only once using published protocols, without optimization. Polymorphism was not detected at locus Ttr48 on the polyacrylamide gel, and this locus was therefore not included in this study. However, this locus might show variability if tested on more individuals or might have alleles that differ in size by only a few repeat units which could not be visualized with the resolution of a 6% polyacrylamide gel. Ttr34 was polymorphic but did not amplify once it was labeled with the fluorescent dye D2 and was dropped.