The tip of the iceberg: Genome wide marker analysis reveals hidden hybridization during invasion

Abstract

Biological invasions are accelerating, and invasive species can have large economic impacts as well as severe consequences for biodiversity. During invasions, species can interact, potentially resulting in hybridization. Here, we examined two Cakile species, C. edentula and C. maritima (Brassicaceae), that co‐occur and may hybridize during range expansion in separate regions of the globe. Cakile edentula invaded each location first, while C. maritima established later, apparently replacing the former. We assessed the evidence for hybridization in western North America and Australia, where both species have been introduced, and identified source populations with 4561 SNPs using Genotype‐by‐Sequencing. Our results indicate that C. edentula in Australia originated from one region of eastern North America while in western North America it is probably from multiple sources. Cakile maritima in Australia is derived from at least two different parts of Europe while the introduction in western North America is from one. Although morphological evidence of hybridization is generally limited to mixed species populations in Australia and virtually absent elsewhere, our genetic analysis revealed relatively high levels of hybridization in Australia (58% hybrids using Admixture) and supported the presence of hybrids in western North America (16% hybrids using Admixture) and New Zealand. Hybrids might be commonly overlooked in invaders, as identification based solely on morphological traits may represent only the tip of the iceberg. Our study reveals a repeated pattern of invasion, hybridization and apparent replacement of one species by another, which offers an opportunity to investigate the role of hybridization and introgression during invasion.

Pollen competition in hybridizingCakilespecies: How does a latecomer win the race?

“…We developed species‐specific cleaved amplified polymorphic sequence (CAPS) markers to assess paternity in pollen competition experiments between the two species C. edentula (2 n = 18) and C. maritima (2 n = 18). Single nucleotide polymorphisms (SNPs) that were potentially fixed between the species were selected based on the allelic frequencies of SNPs derived from the native ranges of C. edentula and C. maritima (where the species are allopatric) using genotype by sequencing data (Rosinger et al, 2021). From the 98 divergent SNPs that were identified between C. edentula and C. maritima , 16 candidate fragments with potential restriction enzymes that could cut one specific allele over another at the SNP site were found.…”

“…In regions where C. edentula and C. maritima coexist, heterospecific pollen transfer is frequent and the chance of hybridization is substantial (Mesgaran et al, 2016). Experimental greenhouse studies have shown that C. maritima and C. edentula are highly crossable when using either species as the pollen donor (Rodman, 1974; Li et al, 2019, 2021); hybrids in both directions of crosses have also been observed in natural Cakile populations in Australia (Rodman, 1974; Ohadi et al, 2016; Rosinger et al, 2021). Previous studies on the hybridization outcomes of Cakile species revealed that pollen viability was equal in C. edentula and C. maritima (Li et al, 2019).…”