Hirotami T. Imai, Robert W. Taylor, Masao Kubota, Kazuo Ogata, and Masayasu Y. Wada.
Notes on the remarkable karyology of the primitive ant Nothomyrmecia macrops, and of the related genus Myrmecia (Hymenoptera: Formicidae).
Psyche 97(3-4):133-140, 1990.

This article at Hindawi Publishing: https://doi.org/10.1155/1990/91237
CEC's scan of this article: http://psyche.entclub.org/pdf/97/97-133.pdf, 1280K
This landing page: http://psyche.entclub.org/97/97-133.html

The following unprocessed text is extracted automatically from the PDF file, and is likely to be both incomplete and full of errors. Please consult the PDF file for the complete article.

PSYCHE
Vol. 97 1990 NO. 3-4
NOTES ON THE REMARKABLE KARYOLOGY OF THE PRIMITIVE ANT NOTHOM YRMECIA MA CROPS,
AND OF THE RELATED GENUS MYRMECZA
(HY MENOPTERA: FORMICIDAE)
BY HIROTAMI T. IMAI', ROBERT W. TAYLOR~, MASAO KUBOTA~, KAZUO OGATA~ AND MASAYASU Y. WADA~
Nothomyrmecia macrops Clark (subfamily Nothomyrmeciinae) is arguably the most primitive of all known ants, and study of this "living fossil" has been deemed "one of the principal challenges of modern Australian entomology" (Brown and Wilson, 1959). Its first two worker representatives were collected in southeastern Western Australia in 1932-33 (Clark, 1934), but despite much effort by many naturalists Nothomyrmecia was not encountered again for nearly half a century, until rediscovered in 1977 near Poochera (32'43's; 134'50'E), South Australia (about 1000 km east of the original site) by R. W. Taylor and colleagues (Taylor, 1978). It has not been found again in Western Australia, and the known distribution at Poochera lies within a radius of only about 1 kilometre. Reasons for this resistance to collection by Nothomyrmecia include its purely nocturnal above-ground worker activities, which are further restricted to relatively cold nights, its cryptic nesting habits, and probably very patchy distribution. Aspects of its anat- National Institute of Genetics, Mishima, Shizuoka-ken 41 1, Japan. division of Entomology, CSIRO, Canberra, Australia, 2601. 3Nakasone 13, Odawara, Kanagawa-ken 250, Japan. "hamiti Institute for Applied Animal Reproduction, Omiya, Saitama-ken 331, Japan.
Manuscript received by the editor May 8, 1990. Pache 97:l-U.l.K~ t HIM). http:ffp~yche.cnli-lub.or~Ìö7 133 hlml



================================================================================

134 Psyche [VOI. 97
omy and behaviour (Taylor, 1978; Holldobler and Taylor, 1983) imply that N. macrops could reasonably exemplify an early stage in ant evolution following the acquisition of eusociality. Information on the chromosomes of this significant insect is important for analysis of karyotype evolution among the ants. An initial cytological check was expedited by A. D. Bishop and R. H. Crozier (Taylor, 1978). They obtained a 2n count of "about 92" for the worker caste, but, because of the high number and small size of the chromosomes, could not be more precise. The authors were able recently to visit the Poochera site, to col- lect adequate new material, and to obtain more satisfactory chro- mosomal preparations.
The subject Nothomyrmecia colony (code HI89-013) was col- lected on 14 December, 1989. Brains from several prepupal workers (at the stage following spinning of the pupal cocoon, and imme- diately after release of the meconium) were prepared using the air- drying technique of Imai et al. (1977, 1988). A total of 10 well-spread metaphase figures were obtained (2 cells each from indi- viduals nos 1 and 2; 1 from no. 5; and 5 from no. 6). All yielded 2n=94.
Detailed analysis reveals that the diploid karyotype (Fig. 1) com- prises one pair of large, and one of medium-sized, metacentrics (M), one pair of medium-sized subtelocentrics (ST), and 44 pairs of medium or small-size acrocentrics (A) or pseudo-acrocentrics (acro- centrics with extremely elongated heterochromatic short arms) (A") - i.e. 2 K = 4 M i- 2 ST 4- 88A or A". All cells examined had the same homomorphic karyotype, and no chromosomal polymor- phism~ were detected.
The two highest chromosome numbers previously recorded for families of the order Hymenoptera are 2nz92-94 for the S.E. Asian ant Platythyrea tricuspidata Emery (Formicidae: Ponerinae: Platy- thyreini), and 2n=66 for the North American paper wasp Polistes exclamens Viereck (Vespidae: Polistinae) (Hung et al., 198 1). The Platythyrea count is corrected here by H.T.I. from the erroneous



================================================================================

-
w
Fig. 1. Karyotype of Nothomymecia macrops 2n=94 (scale = IO~itm). 'dl




================================================================================

136 Psyche [vo~. 97
original 2n=96 of Imai, Brown et al., 1984 (the sole preparation has 2 cells with 2n=92,2 with 2n=94).
N. macrops thus shares with Platythyrea tricuspidata the highest chromosome number known for the order Hymenoptera. The situation in Myrmecia Fabricius (subfamily Myrmeciinae), is even more interesting. Myrmecia is the genus considered most closely related to Nothomyrmecia (Taylor, 1978). It has a known range of chromosome numbers 2n=2 to 2~~84. The 2n=2 count is the lowest physically possible in a metazoan, and here unique to animals more advanced than the nematode Parascaris univalens (Goday and Pimpinelli, 1986). It is from an undescribed, chromosomally polymorphic, southeastern mainland Australian species first reported by Crosland and Crozier (1986), and referred to in our studies as Myrmecia (pilosula) n=l, which is known from detailed analysis to have 21152, 3 or 4; n=\ or 2 (Imai and Taylor, 1989). It isa sibling relative of the "jack jumper" Myr- mecia pilosula Smith, the nominate member of the M. pilosula species complex, which currently includes an estimated 6 morpho- logically closely similar but karyologically diverse putative species (with known chromosome counts of 2n=2-4, 8, 9, 10, 15 and 17-32). All of these, except M. pilosula and one other, are taxonom- ically undescribed, and those previously represented in collections have generally been "lumped" under the namepilosula. One of them (2n=8, n=4) was discovered as recently as December 1989, in the vicinity of Denmark (34' 58's; 1 17'21'E), in extreme southern Western Australia, by H.T.I., M.K. and M.Y. W. Chromosomal details of another of these species, now believed nomenclaturally to be "true" pilosula (2n= 17-32}, were reported by Imai et al. (1988). In addition Myrmecia piliventris Smith is known to have the chromosome complement n=2, 2n=4, and to have closely similar relatives with much higher counts (2n=6, n=3 or 4; 2n=34; 2n=64 respectively; Imai and Taylor, 1 986; Imai et al., 1988). The highest Myrmecia count (that of M. brevinoda Forel, a large eastern "bull-dog ant"; Imai et al., 1988) is the next-highest after Nothomyrmecia and Platythyrea tricuspidata in the Hymenoptera. Thus, this single, morphologically compact genus of around 90 recognised Australian "morphospecies" (and one from New Cale- donia) has a range of chromosome numbers almost as great as that known for its taxonomic order. Over one-third of the range of



================================================================================

19901 Imai et a1.- Nothornyrmecia 137
chromosome numbers in the Hymenoptera is covered by the six M. pilosula-complex siblings alone, and almost two-thirds by the 4 species of the M. piliventris group studied to-date! We therefore believe Myrmecia to be the most karyologically diverse of all animal genera, and the Formicidae one of the most diverse of all animal families.
Ninety species previously considered valid in Myrmecia were listed by Taylor (1987), with 150 available names, including putative junior synonyms, many of which are doubtless spurious. In our view there could be more than 200 biological species of the genus in nature, many of which are probably represented by specimens re- clining cryptically in available collections under the names of puta- tively variable morphospecies, or as presumed junior synonyms. This estimate is based on the modest multiplication available names X 1.5 (1 50 X 1.5 = 225). This calculation would be a radical underes- timate if the pilosula-complex statistics are at all representative of the genus at-large. They objectively justify a maximum multiplica- tion factor of 612 = 3 (6 currently known species versus 2 old available names), yielding an overall estimate of 150 X 3 = 450 Myrmecia species. In this light our estimate of 200 species is very conservative, but we consider it realistic. We expect, however, that the pilosula complex will prove to be more chromosomally diverse and species-rich than presently indicated, and this would have the effect of increasing the value of the maximum multiplier used above. Myrmecia and Nothornyrmecia together encompass (1) the whole range of chromosome numbers known for the Hymenoptera; (2) much of the range known for the Class Insecta, where few counts higher than that of Nothornyrmecia are known (among the highest are several butterflies with counts up to 2n=446, reported by White (1973 chap. 12)), and (3) a similar component of the known range for Phylum Arthropoda, where, for example, higher numbers than 2n=94 were reported by White only from the previously mentioned butterflies, and a few decapod crustaceans (highest reported count 2~~376).
Other ants with chromosome counts equalling or exceeding the vespid wasp maximum of 2n=66 are Myrmecia cephalotes (Clark) (2n=66; Imai et al. 1977), M. tepperi Emery (2n=70; Browning 1987), and M. pyriformis Smith (2n=81; Imai et al. 1977); along with Bothroponera rubiginosa (Emery) (Ponerinae: Ponerini) (2n=



================================================================================

138 Psyche [vo~. 97
76; Imai, Baroni-Urbani et al. 1984), and Monomorium latinode Mayr (Myrmicinae: Solenopsidini) (2n=70; Imai, Baroni-Urbani et al. 1984).
A high chromosome number of 2n=92, has been observed in the Mammalia, for the crab-eating water rat Ichthyomys pitteri (Gardner, 1971, Schmid et al., 1988). It is of some interest that both mammals and ants exhibit an extremely wide range of chromosome number variation: 2n=6-92, and 2n=2-94 respectively (for details see Imai, 1988; and Hoshiba et al., 1989). The so called "fusion hypothesis" has long been invoked to explain the evolution of such variations in chromosome number (White, 1973). It suggests that chromosome numbers tend to reduce in evolution, mainly by centric fusion. An alternative model, the "minimum interaction hypothesis" was recently proposed by Imai, Maruyama et al. (1986). Here chromosome numbers as a whole are postulated to increase by centric fission (Robertsonian fission), thus minimizing the genetic risk resulting from reciprocal translocation, the occurrence of which is more likely between large chromosomes in low-numbered karyotypes (n < 12 in ants) than among small ones in high numbered karyotypes (n > 12 in ants) (Imai et al., 1988). We stress that models of karyotype evolution must explain wide- ranging spectra of chromosome numbers among related organisms, like those demonstrated by mammals, or the Formicidae, or among the myrmeciine and nothomyrmeciine ants, including especially to date the Myrmecia piliventris and M. pilosula species complexes. This study was supported by a grant-in-Aid for Overseas Scien- tific Research from the Ministry of Education, Japan (Imai and Taylor) and The Australian Biological Resources Study Participa- tory Program (Ogata). Taylor is an Honorary Research Fellow of CSIRO Division of Entomology.
Chromosome numbers in the related Australian ant genera Nothomyrmecia and Myrmecia cover the whole range for the Order Hymenoptera (Myrmecia 2n=2-84, Nothomyrmecia 2n=94), and much of that for Phylum Arthropoda. Karyological statistics for



================================================================================

19901 Imai et a1.-Nothomyrmecia 139
these and other ants are reviewed. Myrmecia appears to be the most karyologically diverse of all investigated animal genera, and the Formicidae one of the most diverse of all animal families. BROWN, W. L. JR. AND WILSON, E. 0. 1959. The search for Nothomyrmecia. Western Australian Naturalist 7: 25-30.
BROWNING, G. P. 1987. Taxonomy of Myrmecia Fabricius (Hymenoptera: For- micidae). Ph.D. thesis, Dept of Zoology, University of Adelaide, Australia (with permission).
CLARK, J. 1934. Notes on Australian ants, with descriptions of new species and a new genus. Mem. natn. Mus. Vict. 8: 5-20. CROSLAND, M. W. J. AND CROZIER, R. H.
1986. Myrmeciapilosula, an ant with
only one pair of chromosomes. Science 231: 1278. GARDNER, A. L.
1971.
Karyotypes of two rodents from Peru, with a description of the highest diploid number recorded for a mammal. Experimentia 26: 1088-1089.
GODAY, C. AND PIMPINELLI, S.
1986.
Cytological analysis of chromosomes in the two species Parascaris univalens and P. equorum. Chromosoma (Berl) 94: 1-10. H~LLDOBLER, B. AND TAYLOR, R. W. 1983. A behavioral study of the primitive ant Nothomyrmecia macrops Clark. Insectes Sociaux 30: 384-401. HOSHIBA, H., MATUURA, M. AND IMAI, H. T. 1989. Karyotype evolution in the social wasps (Hymenoptera: Vespidae). Jpn. J. Genet. 64: 209-222. HUNG, H. C. F., REED, H. C. AND VINSON, S. B. 1981. Chromosomes of four species of Polistes wasps. Cariologica 34: 225-230. IMAI, H. T.
1988. Centric fission in man and other mammals. In Cytogenetics Of Mammalian Autosomal Rearrangements. Ed. Daniel, A. R. Liss, New York. IMAI, H. T., BARONI-URBANI, C., KUBOTA, M., SHARMA, G. P., NARASIMHANNA, M. N., DAS, B. C., SHARMA, A. K., SHARMA, A., DEODIKAR, G. B., VAIDYA, V. G., AND RAJASEKARASETTY, M. R. 1984. Karyological survey of Indian ants. Jpn. J. Genet. 59: 1-32.
IMAI, H. T., BROWN, W, L. JR., KUBOTA, M., YONG, H. S., AND THO, Y. P.
1984. Chromosome observations of tropical ants in western Malaysia. 11. Ann. rep. natl. inst. genet. (Jpn) 34: 66-69. IMAI, H. T., CROZIER, R. H. AND TAYLOR, R. W. 1977. Karyotype evolution in
Australian ants. Chromosoma (Berl) 59: 341 -393. IMAI, H. T., MARUYAMA, T., GOJOBORI, T., INOUE, Y. AND CROZIER, R. H. 1986. Theoretical bases for karyotype evolution. I. The minimum interaction hypothesis. Am. Nat. 128: 900-920.
IMAI, H. T. AND TAYLOR, R. W.
1986. The exceptionally low chromosome
number n=2 in an Australian bulldog ant Myrmecia piliventris Smith. Ann. Rep. Nat. Inst. Genetics (Jpn) 36, 1985: 59-61. IMAI, H. T. AND TAYLOR, R. W.
1989. Chromosomal polymorphisms involving telomere fusion, centromeric inactivation and centromere shift in the ant Myr- mecia (pilosula) n= I. Chromosoma (Berl). 98: 456-460.



================================================================================

Psyche
[Vol. 97
IMAI, H. T., TAYLOR, R. W., CROSLAND, M. W. J. AND CROZIER, R. H. 1988.
Modes of spontaneous chromosomal mutation and karyotype evolution in ants with reference to the minimum interaction hypothesis. Jpn. J. Genet. 63: 159- 185.
SCHMID, M., FERNANDEZ-BADILLO, A., FEICHTINGER, W., STEINLEIN, C. AND ROMAN, J. J.
1988. On the highest chromosome number in mammals. Cytogenet. Cell. Genet. 49: 305-308.
TAYLOR, R. W. 1978. Nothomyrmecia macrops: a living-fossil ant rediscovered. Science, N.Y. 201: 979-985.
WHITE, M. J. D. 1973.
Animal Cytology and Evolution; 3rd edn. Cambridge University Press, London.




================================================================================