Abstract
Rapid adaptive responses were evident from reciprocal host-plant switches on performance, digestive physiology and relative gene expression of gut serine proteases in larvae of crucifer pest P. brassicae transferred from cauliflower (CF, Brassica oleracea var. botrytis, family Brassicaceae) to an alternate host, garden nasturtium, (GN, Tropaeolum majus L., family Tropaeolaceae) and vice-versa under laboratory conditions. Estimation of nutritional indices indicated that larvae of all instars tested consumed the least food and gained less weight on CF-GN diet (significant at p≤0.05) as compared to larvae feeding on CF-CF, GN-GN and GN-CF diets suggesting that the switch to GN was nutritionally less favorable for larval growth. Nevertheless, these larvae, especially fourth instars, were adroit in utilizing and digesting GN as a new host plant type. In vitro protease assays conducted to understand associated physiological responses within twelve hours indicated that levels and properties of gut proteases were significantly influenced by type of natal host-plant consumed, change in diet as well as larval age. Activities of gut trypsins and chymotrypsins in larvae feeding on CF-GN and GN-CF diets were distinct, and represented shifts toward profiles observed in larvae feeding continuously on GN-GN and CF-CF diets respectively. Results with diagnostic protease inhibitors like TLCK, STI and SBBI in these assays and gelatinolytic zymograms indicated complex and contrasting trends in gut serine protease activities in different instars from CF-GN diet versus GN-CF diet, likely due to ingestion of plant protease inhibitors present in the new diet. Cloning and sequencing of serine protease gene fragments expressed in gut tissues of fourth instar P. brassicae revealed diverse transcripts encoding putative trypsins and chymotrypsins belonging to at least ten lineages. Sequences of members of each lineage closely resembled lepidopteran serine protease orthologs including uncharacterized transcripts from Pieris rapae. Differential regulation of serine protease genes (Pbr1-Pbr5) was observed in larval guts of P. brassicae from CF-CF and GN-GN diets while expression of transcripts encoding two putative trypsins (Pbr3 and Pbr5) were significantly different in larvae from CF-GN and GN-CF diets. These results suggested that some gut serine proteases that were differentially expressed in larvae feeding on different species of host plants were also involved in rapid adaptations to dietary switches. A gene encoding nitrile-specifier protein (nsp) likely involved in detoxification of toxic products from interactions of ingested host plant glucosinolates with myrosinases was expressed to similar levels in these larvae. Taken together, these snapshots reflected contrasts in physiological and developmental plasticity of P. brassicae larvae to nutritional challenges from wide dietary switches in the short term and the prominent role of gut serine proteases in rapid dietary adaptations. This study may be useful in designing novel management strategies targeting candidate gut serine proteases of P. brassicae using RNA interference, gene editing or crops with transgenes encoding protease inhibitors from taxonomically-distant host plants.
Highlights
The large cabbage white, P. brassicae is a global pest of crucifer crops
Less is understood of the shortterm responses in feeding efficiency and digestive physiology of insects like P. brassicae larvae in reciprocal switches between host plants that belong to different plant families
The CF-garden nasturtium T. majus (GN) larvae ate significantly (p 0.05) less, gained the least amount of weight and produced the least amount of fecal matter indicating poor performance of larvae transferred from cauliflower to T. majus, in CF-GN diet (Fig 1A–1C)
Summary
The large cabbage white, P. brassicae is a global pest of crucifer crops It can feed on members of plant families Tropaeolaceae, Capparidaceae, Resedaceae, and Papilionaceae [1]. P. brassicae larvae are mobile and can move to a new host when the natal host-plant (on which oviposition had occurred) is exhausted Such mobility is typically coincident with later developmental instars [14,15]. Rapid speed and low metabolic costs in adapting to a dietary change (especially a host plant from a different plant family) implies physiological dexterity of the insect digestive system that needs to be better understood. Studies are available from long-term host plant transfers in Lepidoptera and other insects that demonstrate altered larval performance, accompanied by complex changes in gut transcriptomes when switched to a different species of host plants [6,16,17]. Less is understood of the shortterm responses in feeding efficiency and digestive physiology (including molecular changes in gene expression of key players like gut serine proteases) of insects like P. brassicae larvae in reciprocal switches between host plants that belong to different plant families
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