3.4 Individual introgression coefficients, heterozygosity, and assignment to hybrid classes
Introgression analyses using the set of species-specific SNPs were used to classify individuals in the Spanish hybrid regions into eight hybrid classes (pure I. elegans , pure I. graellsii , introgressedI. elegans , introgressed I. graellsii , backcross toI. elegans , backcross to I. graellsii , F1and F2 hybrids) (Table 3; Fig. 2C-D). Samples that werea priori morphologically identified as I. elegans were classified in three classes: pure I. elegans , introgressedI. elegans and I. elegans backcrosses. IntrogressedI. elegans represented the larger proportion of individuals (59 out of 65) in both hybrid regions, while only four I. elegansbackcrosses were found (all in the north-west hybrid region) and only two pure I. elegans (one in north-central and one in north-west; Table 3; Fig. 2C-D). Samples a priori morphologically identified as I. graellsii were classified into five classes: pure I. graellsii , introgressed I. graellsii , back to I. graellsii , F1 and F2 hybrids. Introgressed I. graellsii was the category that included the larger proportion of individuals (37 out of 52) in both hybrid regions [note morphologically I. graellsii were not present in the Menorca from the north-east hybrid region], followed by nine F1 and F2 hybrids, five I. graellsii backcrosses in both hybrid regions and one pure I. graellsii in north-central Spain (Table 3; Fig. 2C-D).
To investigate whether the north-west and north-central hybrid regions had a similar hybridization pattern, hybrid class proportions were compared with Z tests using Yates’s corrections for small sample sizes. We found that both regions showed similar hybrid class proportions for all hybrid classes except for the class I. elegans introgressed (that showed a non-significant trend; ꭓ2=3.438, df=1,p =0.063) and for I. elegans backcrossed (ꭓ2=6.181, df=1, p =0.013), where the north-west hybrid region had higher proportions. Additionally, when hybrid class proportions were analyzed by population, we found extensive introgression in most populations and ongoing hybridization (F1 and F2 hybrids) in four localities (Cañas, Perdiguero, Villar and Louro) (Fig. 3A). Three hybridization patterns were detected in the twelve populations: 1) a bimodal pattern in five populations (Arreo, Cañas, Perdiguero, Valbornedo, and Villar); 2) an introgressed pattern in five populations (Mateo, Valpierre, Doniños, Laxe and Xuño); and a unimodal pattern in one population (Louro) (Fig. 3B).