Myrmecia impaternata, Taylor, 2015

Taylor, Robert W., Imai, Hirotami T., Hasegawa, Eisuke & Beaton, Colin D., 2018, A Unique Conjunction: Evidence for Gynogenesis Accompanying Haplodiploid Sex Determination in the Australian Ant Myrmecia impaternata Taylor, Psyche: A Journal of Entomology (2832690) 2018, pp. 1-7 : 2-6

2. Hybridity and Gynogenesis in Myrmecia impaternata

We review here evidence that M. impaternata is a unisexual clonal species of hybrid origin, arguably with a gynogenetic mode of reproduction in which (i) the parthenogenetic production of diploid female eggs by its queens is sperm-dependent, requiring allospecific copulation followed by interaction without fertilisation between their unreduced oocytes and nonconspecific sperm cells and (ii) sperm necessary for this purpose is likely acquired by the gynes through copulation with technically allospecific males produced from eggs laid by impaternata queens and reared in impaternata nests. In gynogenesis the eggs are not fertilised and female offspring are therefore impaternate [ 5 – 8].

Among vertebrates gynogenesis occurs only in some all-female species of fish, salamanders, and lizards [ 5]. There are no known gynogenetic birds or mammals [ 9]. A number of gynogenetic invertebrates have also been reported [ 7]. All known gynogenetic vertebrates originated as interspecific hybrids [ 9] but this likely detail has not been confirmed for all relevant invertebrates. Most gynogen taxa are thought to have had multiple origins and to coexist in multiple, genetically identifiable lineages [ 8, 10].

The unisexual, internally fertilised, live-bearing Amazon molly-fish Poecilia formosa (Girard, 1859) from northeast Mexico and southern Texas is a well-known gynogen. Its females obtain the required sperm by parasitic gonopodial copulation with males of several other related Poecilia species including its putative female hybridic ancestor P. mexicana Steindachner, 1863 [ 11 – 14].

Gynogenesis apparently functions to provide targeted oocytes with sperm-derived centrosomes which replace their developmentally eliminated original oocyte centrosomes [ 15]. The latter are lost during oogenesis in most appropriately investigated eukaryotes [ 5] apparently to circumvent duplication of the ovarian centrosomes by sperm centrosomes acquired at fertilisation [ 15]. Sperm cells are the products of cell-lines which require centrosomes for cell division and each eventual spermatazoan normally carries a retained centrosome [ 5]. Replacement in the females of the lost oocyte centrosomes is essential for later mitotic spindle formation, cell division, and embryonic development. This apparent vital functionality of gynogenesis implies that other insufficiently studied hybrid taxa might also prove to be gynogens: otherwise how (unless they are appropriately atypical) are their lost ovarian centrosomes replaced?

Chromosomes are not involved in gynogenesis: genetic information from male-derived chromosomes is neither accessed nor required [ 8].

The observations of Ross & Normark [ 15] confirm that cytoplasmic fusion of the gamete cells rather than some other arrangement like simple contact (as sometimes previously speculated) occurs during gynogenesis.

3. Karyological Evidence

Most known M. impaternata karyotypes [2:179] comprise two unmatched haploid chromosome sets with 5 and 14 chromosomes, respectively ( Figure 2 (b)).

M. banksi (2n=10) has the diploid configuration 2K B = {8M, 2A}( Figures 2 (a) and 2(a 󸀠 )), and the 5-chromosome set of M. impaternata closely matches one of its haploid sets of chromosomes K Ba ={4M, 1A} ( Figures 2 (a) and 2(c)). Based on this cytological evidence the impaternata n=5 haploid set is denoted K B ={4M, 1A} ( Figure 2 (b)), and affinity with M. banksi is clearly indicated.

The 14-chromosome haploid set of M. impaternata is putatively denoted K P ={1M, 13A} ( Figure 2 (b)). Among the known karyotypes of other pilosula complex species it most closely resembles that of an M. pilosula colony (accession number HI87-130 [ 2]) from Wambrook Creek (36 ∘ 11 󸀠 S, 148 ∘ 56 󸀠 E), NSW, which has the diploid configuration 2K P = {1M/1A, 15A M, 11A} ( Figure 2 (c)). This karyotype is characterised by one A/M heteromorphic pair of chromosomes resulting from an AM-inversion, with the remaining chromosomes either A or A M, with heterochromatic arms acquired by C-band elongation. Because heterochromatic arms are highly variable and of no genetic significance, A M -chromosomes may be considered equivalent to Achromosomes [ 16]. The 2K P karyotype can be described as 2K P ={1M/1A, 26A} and the two gametes expected from 2K P chromosomes are K Pa ={1M, 13A} ( Figure 2 (c 󸀠 )) and K Pb ={14A}. The former is equivalent to the K P ={1M 13A} haploid set of M. impaternata ( Figure 2 (b)), the diploid karyotype of which (2K I) is formulated 2K I ={K B, K P}, which is equivalent to {4M, 1A; 1M, 13A}.

These karyological similarities suggest that the founding parents of M. impaternata were individuals of M. banksi and M. pilosula (or of their close ancestors) with the latter karyotypically close to the Wambrook Creek series.

The above cytological details (with minor differences in C-banding) apply to the 2n=19 impaternata samples reported previously from the Canberra area, from the southern NSW localities Mayfield and Charleyong and from near Armidale in north-eastern NSW [2:179]. A reported 18-chromosome sample (HI 85-373) from Canberra is part of this karyological series, but is believed to have secondarily lost an Achromosome [ 2].

Imai et al. [2:179] listed two impaternata nest series from Yowrie, NSW, with 2n=15 and karyotypes 2K={K B, K P}={4M, 1A; 4M, 6A}. These have the banksi K B ={4M, 1A} haploid chromosome set, but their pilosula -equivalent component differs from that discussed above and illustrated here. Importantly this might indicate that past hybridization between banksi and pilosula with origination of alternative hybrid entities has successfully occurred more than once and that there could be more than one existing allodiploid banksi / pilosula hybrid clone present in south-eastern Australia.

Because sex determination in ants involves arrhenotoky, the reproductive arrangements in M. impaternata differ significantly from those of Poecilia formosa . Unlike all other known gynogens impaternata females (queens and probably at times workers), being hymenopterous insects, regularly lay unfertilised haploid eggs which develop as males (as in all investigated ants). In M. impaternata , these would each carry one or other of the two parental haploid sets of 5 or 14 chromosomes, since they are derived from hybrid maternal cells with the 2n=5+14 karyology illustrated in Figure 2 . Such males effectively represent those of the two parental species, M. banksi and M. pilosula , which initially hybridized to originate M. impaternata . They would not therefore be technically conspecific with the impaternata females in whose reproductive systems their eggs developed. These males would be expected individually to be morphologically identical to those of either M. banksi or M. pilosula , but that has yet to be confirmed.

4. Evidence from Sperm Samples

HTI has compared sperm cells recovered from (i) testes of a male from one of the impaternata nests discussed above ( Figure 2 (b)), with (ii) others dissected from the spermathecae (sperm storage organs) of several dealate impaternata queens from the same nests ( Figure 2 (c)) and (iii) sperm obtained from the testes of an M. banksi male collected from a nest near Nowra NSW ( Figure 2 (a)) [ 1].

The banksi spermatozoa were all morphologically normal, as illustrated, while most of those from the impaternata - associated male and the impaternata queen ( Figures 3 (b) and 3(c)) were apparently abnormal, with reduced head structure, and presumably incapable of effecting fertilisation. A few morphologically normal sperm cells were observed in the impaternata -derived samples: their functionality, if any, is unknown.

Common identity is confirmed by the similarly degenerate condition of these separate spermathecal and testicular sperm samples. They indicate that copulation with sperm transfer had occurred between impaternata females and impaternata -reared males and that the subject sperm is unlikely to have been obtained by parasitic copulation with another free-living related species such as M. banksi , M. pilosula , or the regularly sympatric M. croslandi (where the sperm cells would need to be functional for conspecific fertilisation and so unlikely to be degenerate). M. croslandi has the remarkable karyology n=1 or sometimes n=2, yielding 2n=2, 3, or 4 [ 1, 17, 18] which might preclude it from such affiliation.

The degenerate structure of impaternata -derived spermatozoa might indicate evolutionary modification to remove their capacity for fertilisation (which could otherwise complicate the gynogenetic process [ 14]), while retaining cellular ability to effect gynogenesis. Males in gynogenesis do not transmit genes and are therefore not directly subject to natural selection. The adaptive evolution of degenerate sperm cells suggested here would necessarily be affected by the positive natural selection of impaternata queens whose male offspring produce degenerate sperm. Causal correlation of sperm degeneracy with gynogenesis in this case seems evident.

The circumstances of copulation in M. impaternata are unknown. We posit that mating probably occurs between males and alate gynes in their home nests prior to dispersal of the gynes and that males might not normally depart the nests at all. They would likely also be short-lived after mating, as usual in male ants, thus with very brief seasonal, exclusively in-nest, presence and availability for discovery. This could explain their evident rarity.

5. Genetic Analysis

Genetic comparison of adult males from the subject colonies to investigate their possible duality was attempted by EH who amplified and sequenced DNA from the inappropriately preserved remains of 2 of the 5 specimens dissected for sperm by HTI. The results were limited because of the inadequate material. Further work will be prioritised when more males become available.

6. Special Features of M. impaternata Biology

Poecilia formosa and other gynogenetic animals are dependent for reproduction on related free-living sperm-donor species and are thus unable to extend their geographical ranges or potential evolutionary survival beyond those of their sympatric hosts [ 19]. They must also confront various other biological disadvantages [ 20]. If our hypothesis is correct, these restrictions would not apply to M. impaternata . It would have no apparent requirement for sperm parasitism of free-living consociates because its gynes arguably have the in-built capacity themselves to produce allospecific males destined to become the gynogenetic partners of their female offspring.

7. Instantaneous Speciation of

M. impaternata

The future of a newly mated ant gyne depends on her ability to establish a foundation nest and produce, by oviposition of fertilised diploid eggs, a first brood of workers able to secure her survival and future reproduction. M. impaternata as a species must have been originated by a successful colonyfounding gyne of either M. banksi or M. pilosula which had been fertilised by a male of the opposite species. Her daughter workers and gynes would have had the unmatched allodiploid karyology illustrated in Figure 2 , compromising any normal reproductive future (for example, their gametes would be a statistical 50:50 mix of those of the two parental species). The chance inheritable parthenogenetic production of unreduced diploid eggs and allospecific males by one such hybrid gyne would have originated M. impaternata as a newly evolved hybridic species. We recognise this event as an example of instantaneous biological speciation [ 21].

The evidential production by impaternata queens of two types of males representing those of its parental species is posited as a simple consequence of hybridization in the presence of arrhenotoky.

8. Concluding Remarks

Unisexuality, hybridity, interspecific mating, and matriclinous inheritance are readily observed in Poecilia formosa [ 11 – 13, 22]. Myrmecia impaternata , of which we have excavated many field nests and observed several laboratory colonies ( Figure 1 ), offers no such convenience. Colony composition, general behaviour, and reproductive organisation appear fully consistent with those of other species of the M. pilosula complex, except for the rarity of males [ 1]. Without our fortuitous knowledge of its allodiploid karyology, discovered by HTI in the course of a general cytological survey of Australian ants [ 1, 2], the eccentric nature of this remarkable species might never have been recognised.

There could well be other similarly cryptic undiscovered cases of gynogenesis among ants and arrhenotokous social wasps and bees. They would be characterised by allodiploid chromosomes and the likely production of two classes of males with differing karyology related to that of associated queens and workers and to the hybridic parental species. Such clones are theoretically likely to be unstable and short-lived [ 8].

Voucher specimens of all species mentioned above are in the Australian National Insect Collection, Canberra.

HTI’s karyological investigations followed the procedures detailed by Imai [ 23]. The text follows established protocols for the recognition of hybrid organisms as taxonomic species [ 24, 25].

Figures

FIGURE 1: Myrmecia impaternata, queen, workers, larvae, and a pupal cocoon in a laboratory observation nest (RWT, CDB).

FIGURE 2: Comparison of karyotypes of Myrmecia banksi (a, a󸀠), M. impaternata (b), and M. pilosula (c, c󸀠)(HTI). A = acrocentric, M = metacentric, and NOR = Nuclear Organiser Region.

FIGURE 3: Morphological comparison of sperm cells from a Myrmecia banksi testis (a), an M. impaternata testis (b), and an M. impaternata queen spermatheca (c) (HTI).