Austroplatypus incompertus

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Austroplatypus incompertus
Austroplatypus incompertus female.jpg
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Coleoptera
Infraorder: Cucujiformia
Family: Curculionidae
Subfamily: Platypodinae
Tribe: Platypodini
Genus: Austroplatypus
Species:
A. incompertus
Binomial name
Austroplatypus incompertus
(Schedl, 1968)

Austroplatypus incompertus, a type of ambrosia beetle, is endemic to Australia. They are found in the mesic forests, and subtropical and tropical ecosystems along the east coast of Australia. There are many unique characteristics attributable to the A. incompertus, like their gallery excavation in several Eucalyptus species, their obligate eusocial behavior, their relationship with fungi, and their unique sexual dimorphism. These beetles are one of the only insects that display obligate eusocial behavior. Additionally, their sexually dimorphic traits are of interest, since body size is reversed with males having smaller torsos than female a.incompertus beetles.

Taxonomy

In the research industry, the A. incompertus has been subject to extensive taxonomic reshuffling, with the species being misidentified by identifying the female and male A. incompertus as different species (given the sexual dimorphism that exists). Also, characterization of the mitochondrial cytochrome oxidase 1 gene showed that there was substantial genetic divergence between southern and northern populations in Australia. Overall, there has been an ambiguity regarding the taxonomy of the Austroplatypus incompertus. Upon constructing a phylogenetic analysis of austroplatypus incompertus, it was discovered that this species is unique from other eusocial organisms in that their lineage is younger than eusocial termites but older than eusocial bees. [1]

With the use of genome-wide markers, one study shows that the genus the Austroplatypus incompertus is a part of dispersal limited and resilient to extinction despite low levels of heterozygosity and gene flow. This characteristic of the genus is similar to other eusocial insect species. Additionally from this study, it is seen another species is newly identified as A. incostatus, which would mean another eusocial species was discovered. [1]

Geographic Range

A. incompertus is local to Australia, and has been confirmed to be found in various places around New South Wales. Their range is somewhat limited, extending from Omeo in Victoria and Eden in NSW north to Dorrigo and west to the Styx River State Forest in Northern NSW. [2]

Habitat

Like other ambrosia beetles, A. incompertus lives in nutritional symbiosis with ambrosia fungi. They excavate tunnels in living trees in which they cultivate fungal gardens as their sole source of nutrition. New colonies are founded by fertilized females that use special structures called mycangia to transport fungi to a new host tree. [3] The mycangia of A. incompertus and the specific manner in which the species acquires fungal spores for transport have been studied and compared with the mechanisms used by other ambrosia beetles. [4] Fertilized females begin tunneling into trees in the autumn and take about seven months to penetrate 50 to 80 mm deep to lay their eggs. [5] [4]

1934. Types of ambrosia beetle galleries, pith and wood tunnels. (33885044901).jpg

Host trees

An assessment done by the United States Department of Agriculture (USDA) on unprocessed logs and chips of 18 woody-plant species from Australia discovered A. incompertus in most of them, including: Eucalyptus baxteri , E. botryoides , E. consideniana , E. delegatensis , E. eugenioides , E. fastigata , E. globoidea , E. macrorhyncha , E. muelleriana , E. obliqua , E. pilularis , E. radiata , E. scabra , E. sieberi , and Corymbia gummifera . Unlike most ambrosia beetles, it infests healthy, undamaged trees. [6]

In Australia, the Austroplatypus incompertus is regarded as a pest, given its life cycle, tendencies to excavate galleries into timber, and its relationship with Raffaelea fungi. The Raffaela found on these beetles cause a pencil streaking effect on tinder, which degrades timber quality. [1]

Food resources

Fungal symbiosis

3 main fungal families were found on the ambrosia beetle species and its habitats were Cladosporium sp, Phaeomoniellacae, and herpotichiellacase. Austroplatypus incompertus is one of the few ambrosia beetles that develop in living trees without affecting the health of the tree itself. The primary fungal symbiont, which is a fungus that takes upon a host organism for its resources, [7] for this species is the Raffaela kenti. Females of A. incompertus have specialized pronotal mycangial plates. These plates contained 70 pits meant to house symbionts like R.kentii.  One study showed that the relative location of this ambrosia beetle determines the composition of the fungal species seen on the mycangial plates. [8]

Parasites

Oftentimes when farming fungus, it is hard to prevent parasites from entering the fungal garden. An environment that harbors good fungal growth is often warm, damp and dark. This is also a great environment for bacterial growth to eat the fungus. One major parasite of these fungal gardens are bacteria in the genus Escovopsis. However, beetles often have the defense of using control bacteria such as Streptomyces to inhibit the growth of bacterial parasites. [9]

Morphology and Life cycle

The egg of A. incompertus is about 0.7 mm in length and 0.45 mm wide. It develops through five instars and its head grows from around 0.3 mm wide in the first instar to 0.9 mm wide in the fifth instar. It then pupates and emerges as an adult - 6 mm long and 2 mm wide. The adult has an elongated, cylindrical body typical of other platypodines, and displays sexual dimorphism, with males being the significantly smaller sex, an atypical arrangement among platypodine beetles. Females have elytral declivity adapted for cleaning of galleries and defense. Also, only females exhibit mycangia.

The larvae of the Austroplatypus incompertus grow in 5 distinct instars, all discernable by morphological features and head capsule widths. Using electron micrographs, it was found that the notches on fifth instar larvae are deeply and narrowly notched and not shallow as previously described. Additionally, the fifth instar larval stage is characterized by a specific prothorax design, a separating factor from the closely resembled the Dendroplatypus species.

The maxillary palps, sensory olfactory organs, are three segmented. Prior research had incorrectly described A. incompertus with  4 segmented maxillary palps, which contributed to the frequent taxonomic misidentification of the species.

Austroplatypus incompertus beetles are a sexually dimorphic species. Their differences in morphology are seen in their elytra; females have an abrupt elytral declivity with prominent spikes and males have one that is rounded with smaller spikes. Additionally, females are equipped with mycangia for fungal growth located in the center of the prothorax, which is not seen in males. Contrary to many sexually dimorphic organisms, males are significantly smaller compared to females. Males do not require a longer torso, as they evolved to no longer guard the colony entrances and rather aid in gallery defense, Female A.incompertus beetles, on the other hand, use their abrupt elytral declivity (a sharp downward posterior slope to the torso) and reinforced central and peripheral spines to block gallery entrances and aid in waste shoveling.

Size variation in A.incompertus is consistent with Bergmann’s rule, which states individuals of a species/clade at higher altitudes or latitudes will be larger than those at lower ones. A significant variation in beetle sizes was seen between different eucalyptus species, with the largest beetles being seen in the Eucalyptus delegatensis tree located in the southern ranges of New South Wales and eastern Victoria, and the smallest being seen in the Eucalyptus andrewsii in the Northern tablelands of New South Wales and adjacent areas of Queensland. [10]

Behavior

Social structure

A fertilized female attempts to start a new colony by burrowing deep into the heart of a living tree, eventually branching off and depositing her fungal spores and larvae. [5] When these larvae grow to adulthood, the males leave some time before the females, with an average of five females remaining behind, which quickly lose the last four tarsal segments on their hind legs. [4] [11] The sole entrance to the colony shortly thereafter will be closed by the tree, enclosing the colony. This deformity and physical barrier causes females to remain unfertilized and they participate in maintenance, excavation, and defense of the galleries, propagating the maintenance of the social hierarchy. [11]

Founding A.incompertus females have been observed to create galleries (holes dug into trees to lay and project larvae) in over 19 different species of Eucalyptus trees. The galleries hold up to 100 larvae and eggs, and up to 13 adult females.

Upon dissection of beetles found in the galleries, it was seen that only one female had developed ovaries, visible oocytes, and a filled sperm storage organ,  implying that the rest of the females did not reproduce. There is a linear growth in the number of broods (child bearing females) per colony, signifying a consultation reproductive return. Additionally, A.incompertus invested equally in males and females producing a 1:1 sex ratio. [12]

Parenting behavior

The roles and behaviors of A. incompertus beetles between sexes have been seen to evolve over time. As mentioned before, the role of guarding the gallery was transferred from males to females. More importantly, analyses indicate that this species transitioned from biparental monogamy to exclusive maternal care complemented with lifetime sperm storage. This behavior likely evolved due to the facilitation of securing lifetime monogamy. [12]

Mating

Mating occurs when a single male excavates a nest founding gallery. After mating, the female continues the excavation of her gallery, with the temporary male assisting in gallery maintenance, entrance blocking, and microclimate regulation for stable fungal growth. Mothers of this species are inseminated through the entirety of their lifetime, furthering their need for a monagamous lifestyle. [13] The secondary purpose of males blocking the gallery entrance is to prevent mobile larvae from rolling/leaving the gallery. [10]

Eusociality

A. incompertus is one of the few organisms outside of Hymenoptera (bees and ants) and Isoptera (termites) to exhibit eusociality. Eusocial insects develop large, multigenerational cooperative societies that assist each other in the rearing of young, often at the cost of an individual’s life or reproductive ability. As a result, sterile castes within the colony perform nonreproductive work. This altruism is explained because eusocial insects benefit from giving up reproductive ability of many individuals to improve the overall fitness of closely related offspring.

For an animal to be considered eusocial, it must satisfy the three criteria defined by E. O. Wilson. [14] The species must have reproductive division of labor. A. incompertus contains a single fertilized female that is guarded by a small number of unfertilized females that also do much of the work excavating galleries in the wood, satisfying the first criterion. The second criterion requires the group to have overlapping generations, a phenomenon found in A. incompertus. Finally, A. incompertus exhibits cooperative brood care, the third criterion for eusociality. [15]

The A. incompertus species was seen to refute industry theories regarding the evolution of plastic helpers. Generally, it is assumed that the helper beetles in cooperative breeding generations are molded through the need for ‘fortress defense’ or ‘life insurance’. However, this theory can not explain the behavior of this ambrosia beetle, because workers are not present at the ‘plastic molding’ period of colony foundation. The role of the helper beetle is given by the inherited indirect fitness benefit to staying and aiding in colony maintenance as opposed to being alone in the wilderness.

One characteristic that defines eusociality is the reduction of an individual organism’s life and reproductive potential in order to raise the offspring of others and overall aid in the life of a colony. One study showed that the helper A. incompertus who initially constructed the galleries could live for 10 to 30 years after their first offspring is born. This contrasts the eusocial norm, where breeding individuals are likely to live longer than workers. [12]

Hypotheses for evolution of eusociality

The reasons behind the evolution of eusociality in these weevils are unclear. [4] [16] The benefits to being altruistic come in two ecological modes: “life insurers” and “fortress defenders”. Most Hymenoptera, the large majority of social insects, are life insurers, where eusociality is adapted as a safeguard from decreased life expectancy of offspring. Most termites, as fortress defenders, benefit from working together to best exploit a valuable ecological resource. [17]

From A. incompertus' ecology, fortress defense is likely considering they excavate wood galleries in host trees with just a single entrance. Fortress defense is sufficient to evolve eusociality when three criteria are met: food coinciding with shelter, selection for defense against intruders and predators, and the ability to defend such a habitat. [18] The female that begins the colony bring the weevils' source of food, its symbiotic fungi, to rest in the wood galleries that it excavates. This satisfies the first criterion. Females exhibit noticeably prominent spines on their elytra, and females are the only sex to defend the galleries, possibly satisfying the second criterion. The third criterion is insufficiently studied and demonstrated. The single entrance could potentially show ability to defend, though several commensals and at least one predator have been found residing in colonies. [4]

Successful eusocial A. incompertus colonies do better reproductively than their non-helping counterparts. [4] This could follow the "life insurer" possibility in that benefits to the offspring of a related individual would increase the desire to assist that individual and have a better chance of gene propagation through kin selection. Hymenopterans that follow such life patterns have a sex determination system where, while the females are diploid and pass down only 50% of their genes due to chromosomal crossover during oogenesis, the males are haploid and pass down their entire genome unaltered. This haplodiploidy hypothesis holds that eusociality evolved because diploid sisters are more related to future sisters than they would be to their own offspring. [19] This hypothesis does not hold up for A. incompertus, however, as a study of genetic markers has shown that all adults, male and female, reproductive or worker, are diploid. [16]

It is entirely possible that this organism evolved eusociality and altruistic behaviors in a different manner from those studied in other species, as it is the first in the order Coleoptera to show such behavior. [15] A. incompertus inhabiting a live tree as opposed to a dead one may be the cause for such behaviors. [16] Success of colonies in this species is relatively low (12%) because it is difficult to occupy the living tissue of the trees and initial success of the fertilized female is challenged by an arduous set-up phase. This has led to the hypothesis that eusociality in colonies with a single female assists in maximizing offspring of a related individual. [4] Relatedness of worker females has not been established, however, and it is unclear that eusociality would be able to evolve simply because of this fact. [16] A further expansion of this hypothesis is that given difficulty of colony founding, helper females may remain in hopes of inheriting the colony. [4] Inhabiting a living tree may offer a much more expansive and sustainable colony for the weevil, but doing so requires maintenance of the galleries from a hostile source environment. It is still unclear if the above reasons are enough to have evolved such behavior in the first place, and discovery of monogamy in the species might further lend to the kin selection hypothesis. [16] Understanding sociality in this group is of great importance in the study of the evolution of such systems, given its unique nature in a far-removed organism. [15]

Interactions with Humans

Not only do the beetles excavate galleries in the tree, the beetles’ symbiotic relationship with ambrosia fungus can also interfere with the tree’s health. This can cause infested trees to die, making this beetle a pest. Pyrethroid insecticides can protect trees from ambrosia beetle attacks. Once beetles are inside the tree, they can be difficult to kill, so the timing of insecticide use can be crucial for pest control. Two commonly used pyrethroid insecticides are permethrin and bifenthrin. [20]

See also

Related Research Articles

<span class="mw-page-title-main">Curculionidae</span> Family of beetles

The Curculionidae are a family of weevils, commonly called snout beetles or true weevils. They are one of the largest animal families with 6,800 genera and 83,000 species described worldwide. They are the sister group to the family Brentidae.

<span class="mw-page-title-main">Telephone-pole beetle</span> Species of beetle

The telephone-pole beetle is a beetle native to the eastern United States and the only living representative of the otherwise extinct family Micromalthidae. Larvae of the beetle live in decaying wood and can be pests to wooden structures, lending them their common name, the 'telephone-pole beetle.'

Ambrosia beetles are beetles of the weevil subfamilies Scolytinae and Platypodinae, which live in nutritional symbiosis with ambrosia fungi. The beetles excavate tunnels in dead or stressed trees into which they introduce fungal gardens, their sole source of nutrition. After landing on a suitable tree, an ambrosia beetle excavates a tunnel in which it releases its fungal symbiont. The fungus penetrates the plant's xylem tissue, extracts nutrients from it, and concentrates the nutrients on and near the surface of the beetle gallery. Ambrosia fungi are typically poor wood degraders, and instead utilize less demanding nutrients. Symbiotic fungi produce and detoxify ethanol, which is an attractant for ambrosia beetles and likely prevents growth of antagonistic pathogens and selects for other beneficial symbionts. The majority of ambrosia beetles colonize xylem of recently dead trees, but some colonize stressed trees that are still alive, and a few species attack healthy trees. Species differ in their preference for different parts of trees, different stages of deterioration, and in the shape of their tunnels ("galleries"). However, the majority of ambrosia beetles are not specialized to any taxonomic group of hosts, unlike most phytophagous organisms including the closely related bark beetles. One species of ambrosia beetle, Austroplatypus incompertus exhibits eusociality, one of the few organisms outside of Hymenoptera and Isoptera to do so.

<span class="mw-page-title-main">Mycangium</span>

The term mycangium is used in biology for special structures on the body of an animal that are adapted for the transport of symbiotic fungi. This is seen in many xylophagous insects, which apparently derive much of their nutrition from the digestion of various fungi that are growing amidst the wood fibers. In some cases, as in ambrosia beetles, the fungi are the sole food, and the excavations in the wood are simply to make a suitable microenvironment for the fungus to grow. In other cases, wood tissue is the main food, and fungi weaken the defense response from the host plant.

<span class="mw-page-title-main">Sociality</span> Form of collective animal behaviour

Sociality is the degree to which individuals in an animal population tend to associate in social groups (gregariousness) and form cooperative societies.

<span class="mw-page-title-main">Eusociality</span> Highest level of animal sociality a species can attain

Eusociality is the highest level of organization of sociality. It is defined by the following characteristics: cooperative brood care, overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups. The division of labor creates specialized behavioral groups within an animal society which are sometimes referred to as 'castes'. Eusociality is distinguished from all other social systems because individuals of at least one caste usually lose the ability to perform behaviors characteristic of individuals in another caste. Eusocial colonies can be viewed as superorganisms.

<i>Coccotrypes dactyliperda</i> Species of beetle

Coccotrypes dactyliperda, the date stone beetle, palm seed borer, or button beetle, is an insect belonging to the subfamily bark beetles (Scolytinae). It originates from Africa and is spread around the world due to the trading and transportation route.

<i>Xyleborus glabratus</i> Species of beetle

Xyleborus glabratus, the redbay ambrosia beetle, is a type of ambrosia beetle invasive in the United States. It has been documented as the primary vector of Raffaelea lauricola, the fungus that causes laurel wilt, a disease that can kill several North American tree species in the family Lauraceae, including redbay, sassafras, and avocado.

<i>Austroplatypus</i> Genus of beetles

Austroplatypus is a genus of weevils native to Australia that includes Austroplatypus incompertus, the first beetle to be recognized as a eusocial insect. Members of the genus live in eucalypt trees and their fungal galleries can persist for decades because the host tree is not being killed.

<i>Kladothrips</i> Genus of thrips

Kladothrips is a genus of Australian gall thrips. It is notable for including some of the few organisms outside of Hymenoptera that exhibit eusociality.

<i>Platypus apicalis</i> Wood-boring beetle endemic to New Zealand

Platypus apicalis, known by its common name the New Zealand pinhole boring beetle, is a wood-boring beetle endemic to New Zealand and found throughout the North and South Island in a range of environments.

<i>Platypus cylindrus</i> Species of beetle

Platypus cylindrus, commonly known as the oak pinhole borer, is a species of ambrosia beetle in the weevil family Scolytinae. The adults and larvae burrow under the bark of mature oak trees. It is native to Europe.

<i>Xylosandrus germanus</i> Species of beetle

Xylosandrus germanus, known generally as the alnus ambrosia beetle or black stem borer, is a species of ambrosia beetle in the family Curculionidae. X. germanus poses challenges in woody ornamental species and orchard crops such as apples and pecan. Furthermore, X. germanus is recognized or suspected to act as a vector for plant pathogens to varying extents, potentially leading to the decline of trees. The black stem borer is native to eastern Asia, but is an invasive species in Europe and North America. This species carries and feeds on associated ambrosia fungus, Ambrosiella grosmanniae.

<i>Xyleborinus saxesenii</i> Species of beetle

Xyleborinus saxesenii, commonly known as the fruit-tree pinhole borer, is a species of ambrosia beetle in the family Curculionidae. It is native to the Palaearctic region but has been introduced in many locations, including North America. X. saxesenii typically live in freshly dead wood, but it has also been reported to attack live trees. Such attacks on live trees may lead to economic damage.

Platypus quercivorus, the oak ambrosia beetle, is a species of weevil and pest of broad-leaved trees. This species is most commonly known for vectoring the fungus responsible for excessive oak dieback in Japan since the 1980s. It is found in Japan, India, Indonesia, New Guinea, and Taiwan.

Euplatypus parallelus, previously known as Platypus parallelus, is a species of ambrosia beetle in the weevil family Curculionidae. The adults and larvae form galleries in various species of tree and logs. It is native to Central and South America but has spread globally, is present in Africa and is well established in tropical Asia.

<i>Euwallacea interjectus</i> Species of beetle

Euwallacea interjectus is a species of ambrosia beetle in the species complex called Euwallacea fornicatus. It is native to Asia but has been introduced to the Western hemisphere over the last century.

Euwallacea perbrevis, commonly known as tea shot-hole borer, is a species of weevil native to South and South-East Asia through to Australia, but introduced to Western countries.

Ambrosiella roeperi is the fungal symbiont of the granulate ambrosia beetle, Xylosandrus crassiusculus, facilitating this insect’s capacity to accumulate on and damage a diverse array of woody plants from around the world. It is one of several important nutritional partners derived from order Microascales that sustain and are transported by xylomycetophagous scolytine beetles.

<i>Euwallacea validus</i> Species of beetle

Euwallacea validus is a species of Euwallacea beetle native to Asia. The beetle species was discovered in Long Island, New York in 1975. Like other Euwallacea species beetles, E. validus is known for its mutualistic symbiotic relationship with fungi, acting as a vector for Fusarium oligoseptatum and Raffaelea subfusca, often using Tree of Heaven as a preferred host. Out of the five confirmed species of Euwallacea spp. in the United States, E. validus is the most widespread and longest established, yet much about their second fungal partner, Raffaelea subfusca, is not known.

References

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