Variable outcomes of hybridization between declining Alosa alosa and Alosa fallax

Abstract

Hybridization dynamics between co‐occurring species in environments where human‐mediated changes take place are important to quantify for furthering our understanding of human impacts on species evolution and for informing management. The allis shad Alosa alosa (Linnaeus, 1758) and twaite shad Alosa fallax (Lacépède, 1803), two clupeids sister species, have been severely impacted by human activities across Europe. The shrinkage of A. alosa distribution range along with the decline of the remaining populations' abundance threatens its persistence. The main objective was to evaluate the extent of hybridization and introgression between those interacting species. We developed a set of 77 species‐specific SNP loci that allowed a better resolution than morphological traits as they enabled the detection of hybrids up to the third generation. Variable rates of contemporary hybridization and introgression patterns were detected in 12 studied sites across the French Atlantic coast. Mitochondrial markers revealed a cyto‐nuclear discordance almost invariably involving A. alosa individuals with an A. fallax mitochondrial DNA and provided evidence of historical asymmetric introgression. Overall, contemporary and historical introgression revealed by nuclear and mitochondrial markers strongly suggests that a transfer of genes occurs from A. fallax toward A. alosa genome since at least four generations. Moreover, the outcomes of introgression greatly depend on the catchments where local processes are thought to occur. Undoubtedly, interspecific interaction and gene flow should not be overlooked when considering the management of those species.

The genic view of hybridization in the Anthropocene

“…Genomic sequencing methods, such as RADseq, cover a larger proportion of the genome compared to microsatellites and can be used to develop diagnostic markers. For instance, Taillebois et al, ( 2020 ) developed 77 species‐specific SNPs (single nucleotide polymorphisms) that could detect hybrids and backcrosses between allis shad ( Alosa alosa ) and twaite shad ( A. fallax ) up to the third generation. Similar marker sets and sequencing protocols have been developed for other study systems (Feulner & Seehausen, 2019 ; Vaux et al, 2021 ; Wielstra et al, 2016 ) and show that conservationists do not always need whole genome sequencing data to reconstruct the entire genomic landscape.…”

Genetic Admixture in the Population of Wild Apple (Malus sieversii) from the Tien Shan Mountains, Kazakhstan

“…Inter-specific gene flow between rare species and the allied taxa that are commonly distributed may result in the increased extinction risk at least locally for the rare species [ 1 , 2 , 3 , 4 ]. Hybridization may lead to increased extinction risk through demographic swamping, i.e., reduced population growth rate, driven by outbreeding depression and/or genetic swamping where the parental lineages are replaced by the hybrids [ 2 , 5 ].…”

Genotype‐by‐environment interactions drive the maintenance of genetic variation in a Salmo trutta L. hybrid zone

“…Mounting evidence indicates that these management practices generally fail at conserving native diversity after admixture had already occurred (Gil et al, 2016 ; Muñoz‐Fuentes et al, 2007 ; Vincenzi et al, 2012 ). This is possibly due to the lack of knowledge regarding reproductive isolation mechanisms (Bajec et al, 2015 ; Berrebi, Poteaux, et al, 2000 ; Iacolina et al, 2019 ; Taillebois et al, 2020 ), which could help to build more efficient and evolutionary‐inspired management strategies. Identification of both pre‐ and postzygotic reproductive barriers should thus be a prerequisite to any management decisions to avoid counter‐productive results.…”