Of “Host Forms” and Host Races: Terminological Issues in Ecological Speciation

Abstract

Successful communication and accurate inferences in science depend on the common understanding and consistent usage of the terms we apply to concepts of interest. Likewise, new terminology is required when important concepts have gone unnamed. Here, I focus on terminological issues about biological variation and ecological speciation, especially in herbivorous insects but also more generally. I call for the more restricted use of concepts that have sometimes been misapplied, and thus caution against synonymizing ecological speciation with sympatric speciation and the unwarranted invocation of "host races" to describe herbivorous insect differentiation. I also call for the qualified application of terms for different kinds of biological variation and for host range when confronting uncertainty. Among other "missing terms" introduced here is "host form," a generic term describing any case of host-associated differences for which current evidence does not allow diagnosis of the specific kind of variation. Embracing the use of host form should free host race from its current overapplication. Finally, I present a case study in which Neochlamisus leaf beetle populations previously described as host forms are hereby declared to be host races, based on accumulated evidence supporting each of the associated criteria.

Distribution and Differentiation ofHenosepilachna diekei(Coleoptera: Coccinellidae) on Two Host-Plant Species Across Java, Indonesia

“…As Funk (2012) recently emphasized, sympatric populations of a phytophagous insect species with divergent host speciÞcity have not necessarily evolved under sympatric conditions and thus do not always support the plausibility of speciation under sympatric conditions. Indeed, the divergent host speciÞcity of H. diekei that was Þrst documented in the sympatric populations in West Java , Matsubayashi et al 2011S.…”

“…In addition, recent renewed interest in the contribution of natural selection to establishing reproductive isolation has given rise to the concept of ecological speciation (e.g., Schluter 2001Schluter , 2009Funk et al 2002;Dieckmann et al 2004;Rundle and Nosil 2005;Funk and Nosil 2008;Nosil et al 2009b;Matsubayashi et al 2010;Nosil 2012). Although diversiÞcation of phytophagous insects through divergent host-plant specialization might occur in any geographical context, including allopatry, parapatry, or sympatry (Matsubayashi et al 2010, Funk 2012, Nosil 2012, the intensity and consequences of divergent natural selection on different host plants can vary depending on the geographical relationship between conspeciÞc insect populations and the degree of host speciÞcity of each population, in other words, the degree of gene ßow between populations on different hosts (Nosil and Crespi 2004;Nosil 2009Nosil , 2012. Hence, elucidating the geographical context in the early phase of divergent host use by phytophagous insects is important to better understand the processes of diversiÞcation in these insects and of ecological speciation (Bush and Butlin 2004).…”

“…Through intensive studies of H. diekei populations in West Java , Matsubayashi et al 2011S. N., unpublished data), it has gradually become evident that the populations occurring on M. micrantha and L. lavandulifolia should be regarded as host races in the broad sense, that is, they are conspeciÞc sympatric populations partially reproductively isolated by divergent host speciÞcity, or "host forms," as recently deÞned by Funk (2012); for an exact deÞnition of host races, see also Drè s and Mallet (2002). Thus, the populations of H. diekei on M. micrantha and L. lavandulifolia in West Java are in an incipient phase of diversiÞcation via specialization on different host plants (Via 2001(Via , 2009Drè s and Mallet 2002;Bush and Butlin 2004;Futuyma 2008).…”

Population structure of Spodoptera frugiperda maize and rice host forms in South America: are they host strains?

“…This choice and adaptation to a reduced number of host plants may cause reproductive isolation in which the final outcome can be the formation of new species (Walsh, 1864;Bush, 1969;Schluter, 2001;Funk et al, 2006;Feder et al, 2012). After this definition, several papers have been published reporting the existence of host races in many insect species with few clear examples in which their existence has been recognized; e.g., the apple maggot fly Rhagoletis pomonella (Walsh) (Walsh, 1867;Bush, 1969;Feder et al, 1994Feder et al, , 2012, the larch budmoth Zeiraphera diniana (Guen ee) (Emelianov et al, 1995;Dr es & Mallet, 2002), and the leaf beetle Neochlamisus bebbianae (Brown) (Funk, 1998(Funk, , 2012. According to Dr es & Mallet (2002), host races can be defined as genetically differentiated sympatric populations that are incompletely reproductively isolated with an appreciable rate of gene flow; they exhibit host fidelity by the use of different host taxa in the wild, display a correlation between host choice and mate choice, and have higher fitness on natal than alternative hosts (Dr es & Mallet, 2002).…”

Reproductive Isolation between Host Races of Phytomyza glabricola on Ilex coriacea and I. glabra

“…Populations seemingly in the midst of divergence and speciation [10], [11], such as host races [10]–[24], provide valuable case studies for the evolution of the genome during speciation. Host races imply genetically distinct populations that are associated with different hosts, such as in herbivorous insects [15], [18], [23], [25]–[30], parasites [31], [32], and parasitoids [33], [34].…”