Evolution, 56(1), 2002, pp. 70±83 POPULATION AND COLONY GENETIC STRUCTURE OF THE PRIMITIVE TERMITE MASTOTERMES DARWINIENSIS MICHAEL A. D. GOODISMAN1,2 AND ROSS H. CROZIER1 1Department of Zoology and Tropical Ecology, James Cook University, Townsville, Queensland 4811, Australia Abstract. The termite Mastotermes darwiniensis is the sole extant member of its family and occupies the basal position in the phylogeny of the eusocial order Isoptera. In this study, we investigated the micro- and macrogeographic genetic structure of M. darwiniensis in its native range in Australia. A total of 1591 workers were sampled from 136 infested trees in 24 locales. Each locale was separated by 2±350 km, and these locales were found within two broader geographic regions approximately 1500 km apart. The multilocus genotypes of all termites were assayed at six polymorphic microsatellite loci. The genetic data indicated that colonies typically fed on multiple trees within locales and extended over linear distances of up to 320 m. Single colonies were frequently headed by multiple reproductives. Workers were highly related (r 5 0.40) and substantially inbred (f 5 0.10). Thus, M. darwiniensis colonies are characterized by the input of alleles from multiple reproductives, which sometimes engage in consanguineous matings. Our analyses of population genetic structure above the level of the colony indicated that locales and regions were signi®cantly dif- ferentiated (ulocale 5 0.50, uregion 5 0.37). Moreover, locales showed a pattern of genetic isolation by distance within regions. Thus, M. darwiniensis populations display restricted gene ¯ow over moderate geographic distances. We suggest that the genetic patterns displayed by M. darwiniensis result primarily from selective pressures acting to maintain high relatedness among colonymates while allowing colonies to grow rapidly and dominate local habitats. Key words. gene ¯ow, inbreeding, Isoptera, microsatellite, polygyny, relatedness, social insect. Received May 7, 2001. Accepted September 4, 2001. Eusocial insects exhibit a variety of remarkable behaviors lifetime (Nalepa and Jones 1991) and may possess greater associated with group living, such as cooperation in rearing plasticity in their developmental and reproductive programs offspring and partitioning of reproduction among colony than the Hymenoptera (Noirot and Pasteels 1987). members (Wilson 1971). The development of many of these A particularly important isopteran candidate for study is traits arose via kin selection, the effectiveness of which de- the giant termite Mastotermes darwiniensis, the sole extant pended on patterns of genetic structure in populations (Ham- representative of the family Mastotermitidae. Fossil evidence ilton 1964). Consequently, many studies have investigated indicates that the family was once distributed worldwide, but the population genetic structure of eusocial insects to better it is now con®ned to Australia and New Guinea (Watson and understand the factors affecting the evolution of sociality Gay 1991). Mastotermes darwiniensis occupies the phylo- (reviewed by Crozier and Pamilo 1996; Pamilo et al. 1997; genetically basal position in the Isoptera and is distantly re- Ross 2001). lated to all other termite families (Thompson et al. 2000). However, the vast majority of these studies have focused The species possesses morphologically primitive character- on eusocial members of the Hymenoptera (all ants, some istics, such as the structure of its hind wings (Watson and bees, and some wasps). The emphasis on this order may bias Gay 1991) and its mechanism of egg-laying (Nalepa and Lenz our understanding of the circumstances under which social 2000), yet its social system is considered to be derived (Wat- behaviors evolve and are maintained, because the Hymenop- son and Gay 1991). tera possess distinct characteristics that may have played a Newly hatched M. darwiniensis larvae develop into one of role in the evolution of sociality in this group. For example, several distinct castes after entering one of two develop- all Hymenoptera are haplodiploid, a condition that alters the mental pathways (Watson et al. 1977; Watson and Sewell genetic relationships within families and may facilitate the 1981). Larvae entering the alate (winged) pathway pass evolution of eusociality (Hamilton 1964; Trivers and Hare through a series of nymphal stages and eventually develop 1976). Thus, investigations of the genetic structure in dis- into sclerotized adults that participate in dispersal ¯ights tantly related eusocial taxa are critical. (Watson et al. 1975; Watson and Abbey 1989). In contrast, Relatively few population genetic studies in the eusocial larvae entering the worker pathway molt into workers and order Isoptera (termites) have been undertaken. This is sur- cannot develop into alates, although they retain several de- prising given the distant phylogenetic divergence between velopmental options. They may go through stationary molts the Hymenoptera and the Isoptera (Kristensen 1991). Indeed, and remain as workers. Alternatively, they may develop into despite the overt similarity of their social systems, the Is- soldiers through a presoldier stage or take on reproductive optera differ from the Hymenoptera in many important ways. roles by molting into neotenics (Watson et al. 1977). Neo- For example, termites are diploid and hemimetabolous tenics are common in old M. darwiniensis colonies (Hill (Krishna and Weesner 1970). They also mate throughout their 1942). They are believed to mate within the nest and replace the primary alate-derived reproductives (Watson and Abbey 1985, 1989). Most mature colonies are headed by hundreds 2 Present address: Department of Biochemistry and Molecular Biophysics, 440 Biological Sciences West, University of Ari- of neotenic reproductives, and alate-derived primary repro- zona, Tucson, Arizona 85721-0088; E-mail: goodisma@email. ductives are virtually never found in natural populations arizona.edu. (Watson and Abbey 1989). 70 q 2002 The Society for the Study of Evolution. All rights reserved. GENETIC STRUCTURE OF MASTOTERMES 71 FIG. 1. Map of northeastern Australia showing approximate locations of 20 locales from which Mastotermes darwiniensis termites were sampled. The species is found throughout the area depicted. New M. darwiniensis colonies are founded by alate repro- ture of M. darwiniensis colonies to identify the systems of ductives following dispersal ¯ights (Watson and Gay 1991). reproduction and recruitment within colonies. We also tried In addition, it has been hypothesized that new colonies may to distinguish colony boundaries and determine the extent to arise by budding off from older established colonies (Hill which individual colonies dominated habitats. Finally, we 1942). Mastotermes darwiniensis colonies do not inhabit con- studied the macrogeographic patterns of genetic structure and spicuous nests. Rather, they live underground or within the attempted to discern the importance of gene ¯ow on larger, food (typically living or dead trees) they consume. Mature regional scales. colonies may nest in many such feeding sites and grow to contain millions of individuals, particularly in disturbed en- MATERIALS AND METHODS vironments where they seem to ¯ourish (Hill 1942). The goal of this study was to obtain a complete under- We collected M. darwiniensis workers from 20 locales sep- standing of the population genetic structure of M. darwin- arated by a minimum of 2 km in the two larger regions of iensis and interpret the results in the broader framework of the Northern Territory and Queensland during the austral sociality in termites. We investigated the sociogenetic struc- winters of 1999 and 2000 (Fig. 1, Table 1). Termites were TABLE 1. Locations (latitude south, longitude east), number of trees sampled (N), and number of Mastotermes darwiniensis workers analyzed (n) from 20 locales in two regions of northern Australia. Locales within regions are numbered in order of increasing south latitude. Region Locale Location Nn Northern Territory N-1 12823.8009, 130855.2479 21 299 N-2 12824.6899, 130855.3669 4 19 N-3 12824.8469, 13189.7699 1 2 N-4 12828.1419, 13183.3659 27 437 N-5 12839.3819, 13184.6019 24 341 N-6 12842.2999, 131824.9779 4 17 N-7 1381.1419, 130856.9419 3 32 N-8 1386.3019, 13186.3819 1 4 N-9 13831.9219, 131820.9609 3 13 N-10 13837.9249, 131837.9599 4 19 N-11 14811.0339, 13282.3799 20 302 N-12 14821.0649, 132826.0659 3 14 N-13 14835.4349, 132810.9209 4 19 N-14 14838.3539, 13287.1849 4 14 N-15 15828.6849, 131823.6099 2 9 Queensland Q-1 14847.2659, 143830.2189 2 8 Q-2 14856.8829, 143833.4279 2 10 Q-3 19814.1489, 146846.2489 2 10 Q-4 19819.6519, 146845.5859 3 5 Q-5 19819.7809, 146843.9509 2 7 Total 136 1591 72 M. A. D. GOODISMAN AND R. H. CROZIER collected from each of several infested trees in each locale, tives descended from a single pair if no more than four alleles and the position of trees within locales was noted. In four were present at any locus. The presence of more than four locales (N-1, N-4, N-5, and N-11, hereafter referred to as alleles at any locus signaled that more than two unrelated ``major locales''), we sampled particularly large numbers of reproductives had produced the sampled workers. workers from many trees. All termites were placed in 95% We next determined if the workers from single colonies ethanol after collection for subsequent microsatellite analy- were clumped within the major locales by examining the sis. patterns of genetic isolation by distance of workers from The genotypes of two to 20 termites from each tree were distinct trees. We ®rst calculated pairwise estimates of FTL assayed at six microsatellite loci, Mdar2, Mdar3, Mdar4, for workers inhabiting different trees within individual lo- Mdar5, Mdar8, and Mdar13, as described by Goodisman et cales. Spearman's rank-order correlation coef®cient (rS) was al. (2001a). To visualize the polymerase chain reaction (PCR) used to measure the correlation between genetic and geo- products, the forward primer for amplifying each locus was graphic distance of trees.