Variation in size and shape of toxin glands among cane toads from native-range and invasive populations

Abstract

If optimal investment in anti-predator defences depends on predation risk, invading new regions (and thus, encountering different predators) may favour shifts in that investment. Cane toads offer an ideal system to test this prediction: expensive anti-predator toxins are stored mainly in parotoid glands whose dimensions are easy to measure, and toad invasions have changed the suites of predators they encounter. Although plasticity may influence parotoid morphology, comparisons between parents and progeny revealed that gland dimensions were highly heritable. That heritability supports the plausibility of an evolved basis to variation in gland dimensions. Measurements of 3779 adult toads show that females have larger glands than males, invasive populations have larger glands than in the native-range, and that parotoid sexual size dimorphism varies strongly among invaded areas. Geographic variation in parotoid morphology may be driven by predation risk to both adult toads and offspring (provisioned with toxins by their mother), with toxins allocated to eggs exacerbating the risk of cannibalism but reducing the risk of interspecific predation. Investment into chemical defences has evolved rapidly during the cane toad’s international diaspora, consistent with the hypothesis that organisms flexibly adjust resource allocation to anti-predator tactics in response to novel challenges.

Intergenerational effects of manipulating DNA methylation in the early life of an iconic invader

“…For example, range-edge toads are more exploratory than those from the range-core [41], which may increase their likelihood of encountering predators. Parotoid glands are smaller in native-range toads than invasive Australian toads, presumably because the toxin is more effective against naive predators in introduced ranges [42]. Range-edge toads in Australia have relatively larger parotoid glands ( [43], but see [42]), which may be a response to increased predation threat to these animals or a more unpredictable predator landscape in newly colonized areas.…”

“…Parotoid glands are smaller in native-range toads than invasive Australian toads, presumably because the toxin is more effective against naive predators in introduced ranges [42]. Range-edge toads in Australia have relatively larger parotoid glands ( [43], but see [42]), which may be a response to increased predation threat to these animals or a more unpredictable predator landscape in newly colonized areas. Fewer and potentially more predictable interactions with predators at the range-core could result in a lower cost of transmitting epigenetic marks to offspring in response to alarm cue exposure, potentially favouring intergeneration effects.…”

Not just the sum of its parts: Geographic variation and nonadditive effects of pyrazines in the chemical defence of an aposematic moth

“…Between‐ and within‐population variation in chemical defences may reflect: (1) solely environmental conditions without any genetic effects, for example, age, local differences in nutrient availability and/or competition for resources that are necessary for the production of the chemical defence (Burdfield‐Steel et al, 2019; Speed et al, 2012); (2) genetic differences in individuals' capacity in sequestering, gathering or synthesizing compounds for chemical defence. In this case, there may be differences between sexes or we may find size‐dependent variation in chemical defence (Alonso‐Mejia & Brower, 1994; Hudson et al, 2021); or (3) that different genotypes may react differently to variable environmental conditions creating genotype‐by‐environment interactions. Evidence for the first mechanism has now been found both in species that sequester their defences (Brower et al, 1982), and in some that produce their defences themselves (i.e., de novo; Burdfield‐Steel et al, 2018).…”

Antimalarial efficacy of Duttaphrynus melanostictus skin extract via inhibition of Plasmodium falciparum Na+/H+ ATPase

“…These results indicated that the toads contain a unique blend of bufadienolides in their skin secretions at each stage of their adult life. There is also a report of ontogenic and regional differences in the composition of toxins in adult toads (Hudson et al 2021). These differential compositions might cause the variation in the therapeutic properties of these toxins.…”