Assortative mating in two populations of Short‐tailed Albatross Phoebastria albatrus on Torishima

Abstract

Assortative mating is an important pre‐mating isolation mechanism that has been observed in some wild populations of seabirds. The Short‐tailed Albatross Phoebastria albatrus is a globally Vulnerable seabird that breeds mainly on Torishima and the Senkaku Islands in the north‐western Pacific Ocean. Our previous studies suggested that two genetically distinct populations exist, one on Torishima and the other on the Senkaku Islands. Recently, however, several un‐ringed birds in subadult plumage have been observed breeding on Torishima in the Hatsunezaki colony. As almost all birds hatched on Torishima since 1979 have been ringed, the natal site of the un‐ringed birds was suspected to be the Senkaku Islands. Genetic differences between the two populations would reveal the natal sites of un‐ringed birds. By observing the ring status (ringed or un‐ringed) of mating pairs and analysing the mitochondrial DNA (mtDNA) control region 2 of un‐ringed birds, we assessed whether birds that originated from Torishima and the Senkaku Islands achieved pre‐mating isolation. There was a small number of pairs on Torishima that consisted of one ringed and one un‐ringed bird, but the observed number was significantly lower than that expected if ringed and un‐ringed birds mated randomly. Furthermore, mtDNA analyses of nine un‐ringed birds demonstrated that all belonged to a particular haplotype clade from the Senkaku Islands. These results show that birds from Torishima and the Senkaku Islands mate assortatively but that there is incomplete pre‐mating isolation between birds from the two island groups. The pre‐mating isolation of these two populations of Short‐tailed Albatross could arise from differences in the timing of breeding and incompatibility in mating displays. As the divergence between the two populations is unlikely to be sufficient to achieve complete post‐mating isolation, the two groups are likely to be hybridizing. Further studies using molecular and/or behavioural analyses would be required to reveal the evolutionary significance of hybridization between these two populations.

Translocation and hand‐rearing result in short‐tailed albatrosses returning to breed in the Ogasawara Islands 80 years after extirpation

“…Previous studies have shown that two populations of STAL existed about 1000 years ago and suggested that descendants of each population have segregated breeding colonies on Torishima and the Senkaku Islands (reviewed in Eda & Higuchi, ). Over recent decades on Torishima there appears to be assortative mating among birds originating from the Torishima and the Senkakus, however successful reproduction did occur when mated pairs included individuals from the two populations (Eda et al ., ). While the historic genetic structure of STALs on Mukojima and the surrounding Ogasawara Islands is unknown, it seems likely that albatrosses on Mukojima were most genetically similar to Torishima, 350 km away, versus the Senkaku Islands, 1850 km away.…”

“…We aligned the obtained sequences with sequences from P . albatrus individuals sampled from Torishima ( n = 51) and the Senkaku Islands ( n = 4) (Kuro‐o et al ., ; Eda et al ., , , ) and archeological Hamanaka 2 site in Rebun Island (45.50°N, 141.07°E) ( n = 46) (Eda et al ., ) using ClustalW in MEGA 6.0 (Tamura et al ., ). Forty‐one of the Torishima samples were from ringed birds that fledged from Torishima, whereas the others were from unringed birds that were suspected to be born in the Senkaku Islands.…”

Documenting the short‐tailed albatross (Phoebastria albatrus) clades historically present in British Columbia, Canada, through ancient DNA analysis of archaeological specimens

“…Currently, albatrosses belonging to Clade 1 are only known to breed on Torishima, while individuals from Clade 2 breed primarily on the Senkaku/Diaoyu/Diaoyutai Islands (Eda et al, 2012 ; Kuro‐o et al, 2010 ). However, due to recent immigration from the Senkaku/Diaoyu/Diaoyutai Islands, a few Clade 2 individuals also now breed on Torishima (Eda et al, 2011 , 2012 ; Eda, Izumi, et al, 2016 ; Kuro‐o et al, 2010 ). In addition to being genetically distinct, stable isotope analysis of archaeological specimens from the Hamanaka 2 site on Rebun Island, Japan, (Figure 1 ) suggests Clade 1 and 2 birds have differing foraging ecologies (Eda et al, 2012 ).…”

“…Analyses of archaeological and modern short‐tailed albatrosses also suggest these clades vary in their morphology, with both their size and beak shape differing (Eda et al, 2012 , 2020 ). The distinctiveness of these breeding populations is maintained by within‐clade assortative mating, which limits gene flow (Eda, Izumi, et al, 2016 ). This assortative mating is believed to be due to prezygotic barriers, including inter‐clade differences in courtship displays and breeding season timing (Eda, Izumi, et al, 2016 ).…”

“…The distinctiveness of these breeding populations is maintained by within‐clade assortative mating, which limits gene flow (Eda, Izumi, et al, 2016 ). This assortative mating is believed to be due to prezygotic barriers, including inter‐clade differences in courtship displays and breeding season timing (Eda, Izumi, et al, 2016 ). Recently, (Eda et al, 2020 ; Yamasaki et al, 2022 ) have argued that the morphological, genetic, and ecological differences between these clades as well as the propensity for within‐clade assortative mating suggest they may in fact represent two cryptic species.…”