Austroplatypus incompertus

Austroplatypus incompertus
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Coleoptera
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.

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 seen 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

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. 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. 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.

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.

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.

Taxonomy

In the research industry, the A. ncompertus 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.

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.

Distribution

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.

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, 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.

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. 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. 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.

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.

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.

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. The secondary purpose of males blocking the gallery entrance is to prevent mobile larvae from rolling/leaving the gallery.

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. 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.

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.

Hypotheses for evolution of eusociality

The reasons behind the evolution of eusociality in these weevils are unclear. 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.

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. 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.

Successful eusocial A. incompertus colonies do better reproductively than their nonhelping counterparts. 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. 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.

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. A. incompertus inhabiting a live tree as opposed to a dead one may be the cause for such behaviors. 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. 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. A further expansion of this hypothesis is that given difficulty of colony founding, helper females may remain in hopes of inheriting the colony. 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. 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.

See also

• Passalidae, an unrelated family of beetles that also live in wood and show sociality

External links

• Jiri Hulcr's Ambrosia Symbiosis Page