We generated 95 complete mitogenomes of T. leucotreta specimens intercepted at import or sampled at outbreak locations. The import specimens represented seven sub-Saharan countries and 12 host plant species across eight genera. These sequences were used to determine the phylogenetic relationship between the specimens and to analyse potential linkages between host plant or country of origin. Based on the these potential linkages we aimed to find an explanation for the occurrence of T. leucotreta interceptions on novel hosts, such as rose.
Mitogenome data generation
With genome sizes in Lepidoptera species ranging from 199 Mbp to 1,253 Mbp [35], our Illumina whole genome shotgun data resulted in assemblies with median size of 1.25 Mbp representing only a small proportion of the entire T. leucotreta genome. However, as the mitochondrial genome is present in multiple copies per cell, the entire mitogenome could easily be assembled even when few nuclear DNA sequences were assembled. Furthermore, even when using small amounts of biological material (e.g. single eggs) as input for the sequencing, library preparation and Illumina sequencing, the complete assembled and annotated mitogenome could be obtained. Insect mitochondrial genomes are highly conserved in Lepidoptera species [36], and the T. leucotreta mitogenome is no exception. The 37 genes typically found in arthropods are present in the mitogenome, i.e. 13 protein coding genes, 22 transfer RNAs and 2 ribosomal RNAs, and the organization and orientation of the mitogenomic components were found conserved relative to seven publicly available complete Olethreutinae mitogenomes.
Genetic structuring between countries
We found no proof of genetic structuring across the seven sub-Saharan countries when comparing the mitogenomes of FCM; all countries were represented by at least two different clades of FCM, with South-African interceptions spanning all six clades. The active dispersal capabilities of FCM could, in theory, be sufficient to have regular mixing of genetic material among neighbouring populations or even countries [37]. However, FCM adults do not usually disperse far when host plants are abundant [9, 38]. This is further supported by regional genetical structuring of South African FCM populations [24] and genetic variation at different altitudes in Kenya and Tanzania [39]. A study on the migratory beet armyworm (Spodoptera exigua) found little genetic structuring among the populations that migrate due to the monsoon season, whereas the clade that undergoes no or little migration had a much higher genetic differentiation and phylogeographic structure [40].
FCM is not known to show migratory behaviour. Therefore, under natural circumstances, we would expect to find a degree of genetic structuring among the countries included in this study. Instead, we find a higher genetic diversity between import consignments from within the same country of origin (e.g. all six clades in South-Africa) than those between the countries. This is in accordance with Mkiga et al. (2021) who found no genetic variability between populations in Tanzania and Kenya [39]. This absence of linkage between genotype and origin suggests that infected fruits and/or plants with soil have been transported between African countries. Citrus, as an example, has been introduced to sub-Saharan Africa on multiple occasions since late 15th century [41-43] and it is very likely to have crossed country borders from there onward, especially because of FCM’s hidden nature as larvae inside the host plant and camouflaged pupae in the soil [13, 14].
Strains linked to different host plants
Our results find no different FCM strains related to any of the 12 host plant species included in this study and Rosa was present as a host plant in all six clades. Moreover, specimens collected from the same host species are often less related to each other from a mitogenomic perspective than specimens collected on other hosts. The lack of linkage between genotype and plant host suggests FCM’s capability to opportunistically switch host taxa, including non-native species such as citrus in the past and Rosa more recently. Many factors influence whether a newly introduced plant can be a suitable host: the phenology of both parties, presence of and resistance against plant defences, landscape composition (e.g. the presence and diversity of native host plants), etc. [44-46]. Generalist and more widespread species are more likely to prefer newly introduced hosts [47] and, as a study on Vanessa cardui suggests, could possess adaptations that allow them to include new plant host species regardless of their phylogenetic relation to already accepted hosts [48]. FCM has many traits to its advantage: a short generation time, the possibility to disperse moderate distances to find suitable hosts, and a broad range of potential host plants in several plant families including a plethora of natural host plants that could support populations during periods when certain crops (e.g. flower buds, young citrus fruits) are absent. FCM has attacked new hosts on a regular basis in the past [17-23] and although its fitness varies between hosts [49], it is clearly able to sustain viable populations on cultivated hosts, possibly shifting to native hosts when needed, without having evolved specific strains related to different host plants.
Almost all samples used in this study were obtained from interceptions at Dutch import locations. Consequently, there is a sampling bias towards countries that trade large volumes of commodities on which FCM is present with the Netherlands [50]; Eastern sub-Saharan countries are well-represented, whereas Western sub-Saharan countries were absent in our samples. Even though the current study does not represent FCM’s complete distribution range, the diversity observed among the south-eastern Sub-Saharan samples is large enough to justify the determination of linkages between mitogenomic diversity and geographic origin or host on which the specimen was found.
This study showcases a unique situation where we are able to document a pest’s early inclusion of a novel host. Large-scale cultivation of flower crops slowly shifted to tropical regions, including Africa during the late 1990s [51, 52]. Our oldest data point dates back to 2013, the first notifications of FCM on Rosa were made in 2007 from several African countries. If host strains of FCM would exist one would expect to find only one haplotype to be associated with this novel host. Instead, we find Rosa as a host in all six clades. Although our data does not focus on Citrus species, another important host taxon of FCM, it is likely that the association with Citrus and other host species is also opportunistic. The ability of larvae to feed on plant parts or developmental stages other than the soft tissue inside fruits is exceptional and was previously not known for T. leucotreta. This stipulates the risks of introducing new crop species to an area; the effect an already present polyphagous pest on the new crop might be unpredicted with current knowledge.