1. INTRODUCTION
Herbivorous insects are often involved in close interactions with their
hosts since they are a food source and provide mating and oviposition
sites (Schoonhoven, Van Loon, & Dicke, 2005). Such intimacy often
entails the evolution of adaptations that allow insects to cope with
specific features of their host plants. Hence, host plant shifts may
affect the evolution of features associated with feeding location,
oviposition and development on the host (Schoonhoven, Van Loon, &
Dicke, 2005; Orsucci, et al., 2018). Adaptation to new hosts may cause,
either as a direct consequence or as a byproduct, the evolution of
sexual isolation, highlighting the role of host plant shifts in
speciation (Funk et al., 2006; Nosil, 2012).
Phytophagous insects with presumptively wide host ranges, i.e.
generalists, pose a concern from an applied view when dealing with
insect pests. Many authors have suggested that polyphagous species are
formed by locally adapted specialized populations or cryptic specialist
species (e.g., Loxdale & Harvey 2016; Forbes et al., 2017). A species
complex made up of cryptic specialists pose an additional complication
in defining host range. The distinction between true polyphagous and
cryptic specialist species is crucial for the design of biological
control programs. In addition, cryptic species complexes offer the
opportunity to investigate the role of host plants in the
diversification of herbivorous insects since such complexes often
include species of recent origin (Winter, Friedman, Astrin, Gottsberger,
& Letsch, 2017; Malka et al., 2018; Bakovic et al., 2019).
Distinguishing between intra and interspecific genetic variation is
particularly relevant in cases of suspected cryptic species, especially
when members of a guild of cryptic species are involved in invasions of
new areas. Invasive insects may cause global problems upon spread to new
territories, not only to agriculture, but also to biological diversity
since invasive species are among the main causes of biodiversity loss
(Newbold et al., 2015). The proper identification of invasive species is
a necessary task to design successful biological control strategies to
prevent or reduce harmful effects of invaders.
With the advent of molecular data, an increasing number of studies
showed that some putative polyphagous insects are, actually, species
complexes embracing several deeply diverged species, each one
specialized to different host plant use (Egan, Nosil, & Funk, 2008;
Nosil et al., 2012; Powell, Forbes, Hood, & Feder, 2014; but see Vidal
et al., 2019). So far most population surveys have been based on the
evaluation of a single genetic marker, the so-called DNA‐barcoding gene
encoding cytochrome oxidase subunit I (mtDNA) (Dinsdale, Cook, Riginos,
Buckley, & De Barro, 2010; Stouthamer et al., 2017). However, it is
well known that in many cases this marker provides information limited
to the mitochondrial lineage, potentially resulting in misidentification
of species boundaries due either to incomplete lineage sorting or
introgressive hybridization (Eyer, Seltzer, Reiner-Brodetzki, & Hefetz,
2017; Després, 2019). Recently, population genetic studies benefited
from availability of new methodologies based on high-throughput
sequencing of genomic libraries containing a reduced-representation of
nuclear genomes, known as genotype by sequencing, such as RADseq (Baird
et al., 2008; Andrews, Good, Miller, Luikart, & Hohenlohe, 2016). These
methodologies provide large multi-locus datasets that can be used to
evaluate cryptic diversity in species complexes (e.g., in Elfekih et
al., 2018), and to trace the origin of invading pests (Anderson, Tay,
McGaughran, Gordon, & Walsh, 2016; Ryan et al., 2019).
The cactus mealybug pest invading Puerto Rico and the adjacent smaller
islands, represents a threat for cactus diversity in the Caribbean,
Central and North America. The pest was initially reported asHypogeococcus pungens Granara de Willink (Hemiptera:
Pseudococcidae), under the common name Harrisia cactus mealybug (HCM),
the successful biological control agent released against an invasive
cactus in Australia and South Africa (McFadyen & Tomley 1978, 1981;
Paterson, Hoffmann, Klein, Mathenge, Neser, & Zimmermann, 2011).
Currently, H. pungens sensu lato is considered a species
complex, and in its native range (South America) it was reported to feed
on species of Amaranthaceae, Portulacaceae and Cactaceae (Ben-Dov, 1994;
Claps & Haro, 2001; Zimmermann, Pérez Sandi Cuen, Mandujano, &
Golubov, 2010). Recent studies based on molecular data and assessing
reproductive compatibility (Poveda-Martínez et al., 2019) and
performance on different hosts plants (Aguirre et al., 2016) suggests
that populations collected in Argentina, initially identified asH. pungens sensu lato , were, actually, a species complex
comprising at least two species; one mealybug species feeding on
Amaranthaceae (H. pungens sensu stricto ), and the other an
undescribed species feeding on cactus. Interestingly, the Puerto Rican
cactus pest appears closer to H. pungens sensu stricto in
phylogenetic trees suggesting that the pest shares a recent ancestor
with the latter rather than with the sympatric, cactus feeding new
species from Argentina. However, none of the mitochondrial haplotypes
found in Argentina matched the single haplotype detected in the Puerto
Rican mealybugs feeding on Cactaceae, suggesting that the source
population of the pest was not Argentina (Poveda-Martínez et al., 2019).
In the present study, we extended the sampling effort to a large
geographic area comprising both the native (Argentina, Paraguay and
Brazil) and non-native (Puerto Rico and southern United States) ranges
by collecting mealybugs on host plants recognized as part of the diet ofH. pungens sensu lato . We used a combination of genome-wide SNPs
and mtDNA variation to investigate: i) genetic diversity within H.
pungens species complex, ii) host plant ranges of each one of the
putative members of the complex; iii) whether host plant shifts drove
the diversification in the species complex, and iv) the source
population of the Puerto Rican cactus pest. Based on these results, we
will be able to search for and select biological control strategies
using natural enemies, either those that co-evolved with the pest
(classical biological control) or those that did not co-evolve but
attack closely related species to the pest (new association biological
control).