High genetic diversity in the clonal aquatic weed Alternanthera philoxeroides in the United States

Abstract

The distribution of genetic diversity in invasive plant populations can have important management implications. Alligator weed (Alternanthera philoxeroides (Mart.) Griseb.) was introduced into the United States around 1900 and has since spread throughout much of the southern U.S. and California. A successful biological control program was initiated in the late 1960s which reduced alligator weed in the southern U.S., although control has varied geographically. The degree to which variation among genotypes may be responsible for variation in control efficacy has not been well studied due to a lack of genetic data. We sampled 373 plants from 90 sites across the U.S. and genotyped all samples at three chloroplast regions to help inform future management efforts. Consistent with clonal spread, there was high differentiation between sites, yet we found six haplotypes and high haplotype diversity (mean h = 0.48) across states, suggesting this plant has been introduced multiple times. Two of the haplotypes correspond to previously described biotypes that differ in their susceptibility to herbicides and herbivory. The geographic distribution of the three common haplotypes varied by latitude and longitude while the other haplotypes were widespread or localized to one or a few sites. All the haplotypes we screened are hexaploid (6n = 102) which may enhance biological control. Future studies can use these genetic data to determine if genotypes differ in their invasiveness or respond differently to control measures. Some states for instance, have mainly a single haplotype and so may respond more uniformly to a single control strategy compared to other states which may require a variety of control strategies. These data will also provide the basis for identifying the source regions in South America, which may lead to the discovery of new biological control agents more closely matched to particular genotypes.

Intraspecific trait plasticity to N and P of the wetland invader, Alternanthera philoxeroides under flooded conditions

“…In the United States, A. philoxeroides has a large geographic distribution that includes the entire southeastern region and California. Of the six recognized haplotypes in the United States, Ap1 has the broadest latitudinal distribution, collected from southern Florida and Texas north to Arkansas and North Carolina (Williams et al, 2020 ). Alternanthera philoxeroides haplotype Ap1 had a similar biomass response at low nutrients as Ap3 and Ap6 but the greatest response to elevated nutrients by most measures, which suggests that it is well‐adapted to a range of nutrient conditions.…”

“…Alternanthera philoxeroides is a common and widespread invader in the Southeastern United States but there are disjunct populations in California (Walden et al, 2019 ). There are at least six haplotypes of alligatorweed in the United States, likely from multiple introductions (Williams et al, 2020 ). The biological control program for alligatorweed consists of three insect agents (alligatorweed thrips, Amynothrips andersoni O'Neill; alligatorweed moth, Macrorrhinia endonephele [Hampson] [= Arcola malloi Pastrana]; alligatorweed flea beetle, Agasicles hygrophila [Selman and Vogt]), which were introduced in the late‐1960s and early‐1970s (Buckingham, 1996 ).…”

“…As early as 1982, it was clear that multiple genotypes of A. philoxeroides were present, with differences in traits between them (Kay & Haller, 1982 ). It has been further acknowledged that multiple introductions likely occurred but, until recently, the degree of genetic diversity and its role in invasive plant management in the United States was unknown (Geng et al, 2016 ; Williams et al, 2020 ).…”

“…Alligatorweed populations used for this experiment were the same as reported by Williams et al ( 2020 ). To determine haplotype, three apical meristems were collected from each culture, dried in silica gel, then DNA was extracted from all samples ( n = 78) using the IBI Scientific MINI Genomic DNA kit (Plants) (Dubuque, Iowa) as per the manufacture's instructions.…”

“…We grew plants under combinations of low or high nitrogen and phosphorus, then measured (1) key growth, structural, and elemental chemistry traits, and (2) short‐term performance of biocontrol agents fed leaves of plants grown under different nutrient combinations. Based on previous studies, we expected the broad‐stemmed morphotype (Ap1 sensu Williams et al, 2020 ) to display the greatest growth response to increased nutrient levels. We additionally expected elevated nutrients to translate into higher tissue nutrients and faster development of the alligatorweed flea beetle.…”