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Breakdown in assortative mating leads to hybrid swarm in pygmy angelfishes
Cohabitation of Indian and Pacific Ocean species at Christmas and Cocos (Keeling) Islands. Coral Reefs
0 T. H. Sinclair-Taylor Red Sea Research Center, Division of Biological and Environmental Science and Engineering, King Abdullah University of Science and Technology , Thuwal 23955 , Saudi Arabia
1 J. P. A. Hobbs (&) J. D. DiBattista Department of Environment and Agriculture, Curtin University , Perth, WA 6845 , Australia
Fig. 1 a A male Centropyge eibli (right) displaying to C. flavissima females (left) minutes before spawning with them. b A male C. eibli (left) and a female C. flavissima · C. eibli hybrid (right) at the start of their spawning ascent
Hybridisation is widespread in coral reef taxa (Richards and
Hobbs 2015); however, the processes leading to interbreeding
are poorly understood. A breakdown in assortative mating is
commonly implicated in hybridisation of reef fishes, and
although mixed-species groups are often recorded, interbreeding
is rarely observed (Hobbs and Salmond 2008). Interbreeding
could occur unintentionally (accidental hybridisation), or
through the deliberate actions of one (sneak spawning) or both
parent species (disassortative mate choice).
Molecular studies reveal a history of hybridisation among
pygmy angelfishes (DiBattista et al. 2016). These fishes form
discrete harems comprised of one large male and multiple smaller
females. During field trips to Christmas Island (Indian Ocean) in
September 2015, April 2016 and June 2017, we observed
interbreeding between Centropyge flavissima, C. eibli and C. vrolikii.
We visited 19 mixed-species harems and observed the male courted
and then sequentially spawned with each of the females in his
harem (Fig. 1a; electronic supplementary material, ESM). Across
the harems we observed every mating combination involving males
and females of each species and their hybrids (Fig. 1b: ESM).
The mixed-species harems we observed have been
stable (same location and harem members) for 1–6 yrs.
Monitoring of six harems revealed interbreeding occurred on
consecutive days and across multiple years. We conclude that
members of the mixed-species harems repeatedly choose to
interbreed with other species. Introgression identified in the
genes of these three species (DiBattista et al. 2016) can be
explained by the ongoing breakdown in assortative mating
caused by disassortative mate choice.
Breakdown in assortative mating leads to hybrid swarm in pygmy