Myrmecia impaternata

Myrmecia impaternata is broadly sympatric with Myrmecia croslandi, a matter of possible biological significance. Both species are common in and around Canberra and on the New England Tableland around Armidale. There are confirmed Queensland records of M. croslandi from the Darling Downs in extreme SE Queensland, and of M. impaternata from Tamborine Mountain south of Brisbane. JACP voucher specimens of this taxon were discussed by Imai, Taylor et al. (1994) as “PBF1 hybrids”, with 2 types: PBF1–1 and PBF1–2 (PB = pilosula x banksi).

Distribution based on Regional Taxon Lists
Australasian Region: Australia.

Biology
Taylor (2015)

All M. impaternata sympatric associations throughout its known distribution are with M. croslandi. Nests of both species are frequently encountered interspersed in Canberra parks and gardens, suburban roadside grass lawn “nature strips” and in local grassy bushland. Colonies of the two have often been encountered by the author and other researchers only a few meters apart.

Reproduction
There has been persistent historical failure by JACP researchers, and the author subsequently, to discover males in M. impaternata nests, despite targeted excavation of field colonies at appropriate seasons over many years. Only two male-right colonies have ever been located and collected. Both were closely adjacent at the Canberra Botanic Gardens.

Research on males and queens from these nests by Taylor, Imai and Hasegawa (to be published elsewhere) has investigated the reproductive biology of M. impaternata. It convincingly demonstrates that M. impaternata is a sperm-dependent gynogenetic taxon in which unreduced eggs require contact with sperm or spermatic fluid (specifically without the occurrence of fertilization) in order to develop parthenogenetically, and thus to produce diploid workers and gynes (see Kokko et al. (2008) for theoretical background). The necessary spermatic material is evidentially obtained by gynes through copulation with nominally conspecific donor males bred in impaternata nests. In this example the sperm cells dissected from male testes and gyne spermathecae were identically and characteristically structurally degenerate and putatively incapable of actually effecting fertilization. Their presence in both sexes importantly attests previous mating between relevant males and gynes. The where’s and when’s of copulation are not known.

In other known sperm-dependent gynogenetic animals (all of which are hybrid-originated allodiploid entities, including various fish and amphibian species, none of which are known to possess a male sex) sperm is obtained for this unusual purpose by the females through parasitic copulation with males of other separate, sympatric, congeneric and usually closely related donor species. The distribution, stable presence and long-term population survival of such gynogenetic taxa is totally dependent on sympatric associations with sperm-donor host species, a factor which critically restrains their distributional ranges and dispersability, but ultimately ensures their survival as species in nature.

Because males in ants are genetically haploid, it is suggested that those produced by impaternata females will likely be of two types, genetically, karyologically and perhaps morphologically equivalent to males of the putative parental species M. banksi and M. pilosula (Eastern Race). Remarkably, males produced in impaternata colonies would technically therefore not be conspecific with their impaternata mothers. Taylor, Imai and Hasegawa conclude that: “M. impaternata thus has no need to maintain risky, restrictive parasitic affiliation or sympatry with other free-living, closely-related sperm-donor host species. It is apparently able to produce the necessary allospecific males by accessing its own genome!” The authors also suggest that M. impaternata queens might at times more usually operate as sperm parasites of M. croslandi by obtaining sperm allospecifically from croslandi males. This hypothesis is encouraged by the persistent historical failure to discover males in M. impaternata nests.

Further understanding of the reproductive biology of M. impaternata is greatly desirable. Of particular interest are: (1) determining whether sperm or spermatic materials actually enter the egg cytoplasm or not; (2) investigation of the possibility that two classes of males (comparable respectively to those found in colonies of M. banksi and M. pilosula [Eastern Race]) are developed from haploid impaternata queen-laid eggs; (3) testing the possibility that production of “impaternata” males and their presence in nests might in this case be unusual; (4) finding whether impaternata might also (more usually?) maintain a parasitic copulatory relationship with M. croslandi, considering their frequent, very proximate, and wide-ranging sympatric co-presence; and (5) determining when and where copulation occurs in either of these scenarios.

Karyology
Details are provided by Imai, Taylor et. al. (1994), and by Taylor, Imai and Hasegawa (in preparation). Myrmecia impaternata has an allodiploid karyotype: n=5 or 14, 2n=19. The 5-chromosome set closely matches one of the haploid sets of Myrmecia banksi, while the 14-chromosome haploid set is generally matched in the Eastern Race of Myrmecia pilosula – most closely resembling chromosomes from a colony (HI87–130) collected at Wambrook Creek (36º11'S, 148º56), near Cooma, NSW. On these grounds M. banksi and M. pilosula (Eastern Race) (or close ancestral stocks) are identified here as the parental species which hybridized to originate M. impaternata. See also Imai, Taylor et al. (1994) appendix and fig. 8 (p. 150).

Nomenclature

 *  impaternata. Myrmecia impaternata Taylor, 2015: 514 (w.) AUSTRALIA.

Description
Despite its apparent hybrid origin and reproduction by theletokous parthenogenesis, M. impaternata functions in nature as a biological species. It is accorded specific taxonomic status here following the precepts of Maslin (1968) and Cole (1985), on the grounds that it is a genetically and historically unique, self perpetuating, separately evolving entity, reproductively isolated from its ancestors and sympatrically-associated related species.

Worker
General features as illustrated and in key couplets 1 – 4, which cover leg-coloration. The brassy color of the cephalic pubescence can be hard to discern stereomicroscopically using some types of illumination lamp, and color temperatures of the light provided. It is best observed beyond the near-side eye in acute diagonal lateral view of the head. This pubescence is arguably a legacy of the Myrmecia banksi hybrid parentage. Otherwise M. impaternata is similar in physiognamy and sculpturation to small/medium–sized workers of the second parental species, Myrmecia pilosula (Eastern Race). There is some size-related graded variation in cephalic and dorsal mesosomal sculpturation between small and large specimens, somewhat as in both races of M. pilosula but less extreme, especially at the high, more intensively-sculptured end of the range.

For significant scientific reasons discussed below investigation of the comparative biology of, and possible reproductive relationships between, M. impaternata and Myrmecia croslandi is a prime scientific subject. The two species may be readily differentiated using the characters of couplet 2 of the key.

(Holotype, smallest paratype, largest paratype (mm): TL = 13.01, 10.87, 13.04; HW = 2.47, 2.22, 2.49; HL = 2.27, 206, 2.27; CI = 108, 107, 109; EL = 0.96, 0.88, 0.97; OI = 39, 39, 39; SL = 1.87, 1.84, 1.94; SI = 76, 82, 78; PW = 1.58, 1.42, 1.59; WL = 3.52, 3.34, 3.75; PetW = 0.97, 0.83, 0.96; PpetW = 1.39, 1.22, 1.33.

Type Material


The type-locality is common also to M. croslandi (see above). The site, also supports M. impaternata, M. croslandi and M. pilosula (Eastern Race).

Holotype and paratypes in, paratypes or type-compared vouchers in , , , , WAMA, TMHA) and in , , ,.

Etymology
The name impaternata is based on the Latinate (not truly Latin) biological term “impaternate” (= fatherless as a result of parthenogenesis). For explanation see the section on reproductive biology below.