The Octodontidae Revisited

Una Revision de Octodontidae

Milton Gallardo, R. Ojeda, C. González, and C. Ríos

ABSTRACT

The monophyletic and depauperate assemblage of South American octodontid rodents has experienced an extensive adaptive radiation from above-ground dwellers to subterranean, saxicolous, and gerbil-like deserticolous life forms. Complex and saltational chromosomal repatterning is a hallmark of octodontid evolution. Recent molecular evidence links these chromosome dynamics with quantum genome size shifts, and probably with reticulate evolution via introgressive hybridization in the desert dwellers Tympanoctomys barrerae and Pipanacoctomys aureus. Genome duplication represents a novel mechanism of evolution in mammals and its adaptive role is reflected in the ability of deserticolous species to colonize the extreme environment of salt flats. Unique to Tympanoctomys is a the rigid bundle of hairs behind the upper incisors which is crucial to efficiently peel saltbush leaves and probably explains its broader distribution relative to P. aureus. This feature, in association with other attributes (e. g., specialized kidneys, large bullae, feeding behavior) has enabled Tympanoctomys to cope with extreme environmental conditions.

Key words: Octodontidae, Octodontids, South American mammals, tetraploidy, Tympanoctomys barrerae.

RESUMEN

Los octodóntidos son un grupo de roedores monofiléticos que han experimentado una extensa radiación adaptativa desde especies que viven en la superficie a formas de vida subterráneas o de tipo gerbos, especializados a la vida desertícola. La evolución de los octodóntidos está marcada por reordenamientos cromsómicos complejos y de tipo saltatorio. Las evidencias moleculares recientes indican una estrecha asociación entre esta dinámica cromosómica, los cambios genómicos cuánticos y la evolución

Pp. xx-xx in Kelt, D. A., E. P. Lessa, J. Salazar-Bravo, and J. L. Patton (eds.). xxxx. The quintessential naturalist: honoring the life and legacy of Oliver P. Pearson. University of California Publications in Zoology xx:xx-xx.

  University of California Publications in Zoology reticulada mediante hibridización introgresiva de los desertícolas Tympanoctomys barrerae y Pipanacoctomys aureus. La duplicación genómica representa un nuevo mecanismo de evolución en los mamíferos y su rol adaptativo se refleja en la habilidad de estas formas desertícolas para colonizar los ambientes extremos de los salares. Un rasgo único de Tympanoctomys son sus cerdas bucales, ubicadas detrás de los incisivos. Esta adaptación es crucial para pelar las hojas de las plantas halofíticas y seguramente constituye la ventaja que explica su mayor distribución en relación a Pipanacoctomys aureus. Este rasgo, asociado a otros atributos ( e.g. riñones especializados, grandes bullas y comportamiento de alimentación) ha permitido la adaptación de Tympanoctomys a estos ambientes de condiciones extremas.

Palabras claves: Octodontidae, Octodontids, mamíferos Sudamericanos, tetraploidía, Tympanoctomys barrerae.

INTRODUCTION

Endemic of South America, octodontid rodents represent a monophyletic, species- poor but ecologically diverse assemblage of genera including above-surface dwellers, deserticolous, fossorial, and strictly subterranean forms (Table 1). Electrophoretic patterns (Köhler et al., 2000), DNA-DNA hybridization data (Gallardo and Kirsch, 2001), and sequencing of nuclear and mitochondrial markers support a basal clade containing the desert specialists, distributed along the eastern slope of the Andes. This clade is genetically distinct from the species distributed along the western slope (Honeycutt et al., 2003). Extensive chromosomal repatterning, absence of intra- or interpopulational chromosome polymorphism and a bidirectional trend of karyotypic evolution are characteristic of the Octodontidae (Gallardo, 1992, 1997). Indeed, a decrease from the modal 2n = 56-58 characterizes Octodontomys gliroides (2n = 38) whereas a saltational increase in diploid number (2n = 102) associated to genome size duplication has been reported for the red vizcacha rat, Tympanoctomys barrerae (Gallardo et al., 2003, 2004a). The recent description of the salt flat dwellers,Pipanacoctomys aureus and Salinoctomys loschalchalerosorum has increased the recognized diversity of the deserticolous octodontids (Mares et al., 2000). The phylogenetic relationships of P. aureus to T. barrerae, its fully biarmed, 92-chromosome karyotype, and its unexpected double genome size provides evidence to advance a hybrid origin for T. barrerae (Gallardo et al., 2004a). Quantum increases of DNA content in the Octodontidae represent a unique trajectory in the group´s diversification and a novel mechanism of genome evolution in mammals. The buccal structures that allowed T. barrerae to feed on halophytic plants, and hence to colonize the arid salt flat formations, reflect the adaptive nature of this novelty. Data at hand have helped our understanding of the groups reticulate history and a shift from the idea that extant octodontids are remnants of a past and wider adaptive radiation (Reig, 1981). All these features make the octodontids a very distinctive group for studying its genome dynamics, and for contrasting macroecological and phylogenetic hypotheses. Here, updated data on the distribution, systematics, and natural history of the Octodontidae are provided, and a model to account for the genome duplication involving T. barrerae and P. aureus is advanced. Gallardo et al.: The Octodontidae Revisited  38° S 27°- 36° S 28°- 33° S 39° - 40º S 30º - 36° S 35° - 36° S 37° - 39° S 34° - 38° S Distribution (Latitudinal Range) ? ? Diet Seeds and bulbs Araucaria seeds, bamboo shots Geophytes, Hemicryptophytes Chenopods, grasses, forbs, roots, seeds, leaves seeds, Leaves, grasses Shrubs, forbs, grasses and forests and forests; forests Habitat Nothofagus Araucaria sandy and xerophytic shrubs Bunchgrass and Nothofagus Nothofagus Araucaria Semidesert sandy loam, pasture lands, Coastal and central scrublands Rocky and dense scrubland Rocky and dense coastal scrublands Mocha Island Habits Semifossorial, colonial burrows Semifossorial Semifossorial Fossorial, colonial burrows Terrestrial, colonial Scansorial Scansorial Scansorial Ecoregion Andean –Patagonian forest Mediterranean, Matorral Andean forest Andean and coastal areas Mediterranean, Matorral Mediterranean, Matorral Mediterranean, Matorral Insular habitats 290 Body 80-110 60 - 110 170-260 155-240 130-235 83 - 110 Mass (g) 123 - 134 SPECIES Aconaemys fuscus Aconaemys sagei Aconaemys porteri Spalacopus cyanus Octodon degus Octodon bridgesi Octodon lunatus Octodon pacificus Table Table 1.- Ecological diversification of living octodontids and their latitudinalRedford distribution2003; al., et ranges.Oyarce 2000; Sources:Muñoz-Pedreros, 1996; al., et ContrerasOjeda 1999; 1991, etNowak, 2000; al., et al.,Mares 1982; Ojeda, and Mares 1987; and Eisenberg, 1992.  University of California Publications in Zoology 27° S 30° S 29°- 39ª S 15° - 28° S 28° - 32° S Cacti, acacia pods, seeds mesquite, shrubs, cacti chenopodes, mesquite leaves chenopodes, mesquite Chenopodes? Rocky slopes; near (pircas) rocky walls Rock outcrops Salt basins; sand dunes Salt basin Salt basin Scansorial, superficial burrows, solitary? Saxicollous, solitary? Ground dwellers, burrow system, solitary Ground dwellers, burrow system, solitary Ground dwellers, burrow system, solitary Highland; Puna and Prepuna Lowland ; Monte desert, Lowland; Monte desert; Monte-Chaco ecotone; Monte –Patagonia ecotone Monte desert Monte-Chaco ecotone ? 107 80-110 81 - 104 115 - 176 Table 1 (continued). Table Octodontomys gliroides Octomys mimax Tympanoctomys barrerae Pipanacoctomys aureus Salinoctomys loschalchalerosorum Gallardo et al.: The Octodontidae Revisited 

BIOGEOGRAPHY, DISTRIBUTION, AND NATURAL HISTORY

The history and evolution of the South American caviomorph rodents has been thoroughly discussed by different authors (Patterson and Pascual, 1972; Reig, 1981; Woods, 1982; and references therein). The most ancestral caviomorph fossil, Platypittamys, known from the Oligocene of Bolivia and Patagonia (Wood and Patterson, 1959; Patterson and Pascual, 1972; Patterson and Wood, 1982) was a generalized ground- dweller lacking the hypsodont teeth that evolved concomitantly as aridity increased (Webb, 1978). The radiation of the octodontids is connected with the Andean orogenesis as exemplified by the major Miocene-Pliocene faunistic diversification and subsequent climatic and vegetational changes (Contreras et al., 1987 and references therein). As the Andes rose up, new wind patterns resulted in different precipitation regimes across the mountain range (Solbrig, 1976; Mares, 1985). While humid forests and scrublands flourished along the western slope of the Andes, a gradual increase in arid conditions took place on its eastern side (Contreras et al., 1987). Central Argentina was already semiarid during the Eocene whereas Patagonia had a humid climate that supported woodlands during early Miocene (Wolkheimer, 1971). The shift to aridity occurred at the Miocene-Pliocene boundary whereas the northern Puna Desert developed as the Andes uplift took place during the Pleistocene (Simpson, 1975). Several families of caviomorph rodents (abrocomids, caviids, chinchillids, and octodontids) evolved to exploit diverse niches in response to the increasing aridity derived from these orogenic events (Mares and Ojeda, 1982; Mares, 1985). The narrow, longitudinal distribution of the Octodontidae ranges from coastal central Chile to pre-Andean or Andean regions of Chile and Argentina. Latitudinally, it ranges from semiarid and desert shrublands to Mediterranean scrublands and humid forests between 15° to 40° S (Fig. 1). The diversification of this family in ecological (Mares and Ojeda, 1982), morphological (Mares et al., 1997), physiological (Díaz and Ojeda, 1999), and behavioral terms (Giannoni et al., 2000; Torres et al., 2003) shows some remarkable examples of convergence with rodents from other deserts (Ojeda et al., 1999). Three major divisions can be recognized in the Octodontidae; an Andean- pre Andean group is represented by the scansorial Octodontomys gliroides (Fig. 1I) and there are 2 mainly lowland groups, the “Chilean” and the “Argentinean.” The “Chilean” forms radiated into fossorial (Aconaemys; Fig 1B-D) and strictly subterranean forms (Spalacopus cyanus; Fig 1K) while another genus diversified into above- surface generalists (Octodon; Fig 1E-G) within Mediterranean scrubland and forests. The “Argentinean” group radiated into saxicolous (Octomys Fig 1J) and burrowing (Pipanacoctomys, Tympanoctomys and Salinoctomys; Figs. 1L-N) species, highly adapted to the desert patchy habitats of salt basins, sand dunes, and rock outcroppings.

SYSTEMATICS

The monophyly of the Octodontidae is well supported and the initial diversification of extant genera is estimated to have occurred at 9 myr bp (Gallardo and Kirsch, 2001). Although the sister taxon and the taxonomic rank of the Octodontidae was debated, the group´s close affinity to Ctenomys has been corroborated by molecular  University of California Publications in Zoology

Figure 1. Geographic distribution of the Family Octodontidae (A) and their species: B) Aconaemys fuscus; C) Aconaemys porteri; D) Aconaemys sagei; E) Octodon bridgesi; F) Octodon degus; G) Octodon lunatus; H) Octodon pacificus; I) Octodontomys gliroides; Gallardo et al.: The Octodontidae Revisited 

J) Octomys mimax; K) Spalacopus cyanus; L) Pipanacoctomys aureus; M) Salinoctomys loschalchalerosorum, and N) Tympanoctomys barrerae. The box in panel A shows the area that is enlarged in subsequent panels.  University of California Publications in Zoology

Figure 1 (continued). Gallardo et al.: The Octodontidae Revisited 

Figure 1 (continued). 10 University of California Publications in Zoology

Figure 1 (continued). Gallardo et al.: The Octodontidae Revisited 11

Figure 1 (continued). 12 University of California Publications in Zoology

Figure 1 (continued). Gallardo et al.: The Octodontidae Revisited 13 data sets (Gallardo and Kirsch, 2001; Honeycutt et al., 2003). Indeed, molecular differentiation supports the family ranks of Ctenomyidae and Abrocomidae and place the latter at the base of the octodontid clade. The clade containing the Echimyidae and the Myocastoridae branches off between the Ctenomyidae and the Abrocomidae (Honeycutt et al., 2003). The robustness and consistency of molecular data provides no evidence for excluding Octodontomys from the Octodontidae sensu stricto, as had been suggested by Verzi (1994). It also suggests that the cranio-mandibular and dental morphology analyses of Verzi (2001) as well as the analysis of penial morphology of Contreras et al. (1993) may bear high levels of homoplasy. Several accounts have stressed the wide range in chromosome number (2n = 38 -102) of the octodontids and its coincidence with the specific status of each form (Contreras et al., 1990; Gallardo, 1992; Spotorno et al., 1995). These extreme differences were interpreted as resulting from chromosome fusions with lower numbers derived from a high-numbered ancestral karyotype (Spotorno et al., 1988). Alternatively, Gallardo (1992, 1997) proposed a birectional trend, with a decrease from the modal 2n = 56-58 as observed in O. gliroides (2n = 38), and a saltational increase in chromosome number in T. barrerae (2n = 102). Combined analysis of mitochondrial and nuclear genes as an interpretative framework to test this hypothesis implied an ancestral 2n between 46 and 56 for the Octodontidae, thus supporting the bidirectional evolution of diploid numbers (Honeycutt et al., 2003) Based on the close allozymic association between 102-chromosome Tympanoctomys and-56-chromosome Octomys,, the saltational model of karyotypic evolution predicted double genome size in T. barrerae (Gallardo, 1997). This was confirmed by genome size estimates although the suggestion advanced for O. lunatus was not supported by the data (Gallardo et al., 2003). The origin of T. barrerae has remained enigmatic since no karyotypic combination of any known octodontid species could originate its 102- chromosome karyotype (Gallardo et al., 1999). The subsequent description of P. aureus (Mares at al., 2000) has helped to resolve the reticulate history of the desert dwellers, and to document that speciation by polyploidy is possible in mammals (Gallardo et al., 2004a). P. aureus has a totally biarmed, 92-chromosome karyotype having several heteromorphic pairs. The Y chromosome is the only acrocentric of the male karyotype. The pair having a secondary constriction is present also in 2 biarmed chromosomes, as reported for T. barrerae (Gallardo et al., 2004a). The genome size of P. aureus is 2C = 15.34 ± 0.67 pg DNA whereas gametic DNA content is 1C = 7.18 ± 0.56 pg in the sperm cells. These estimates, although less than in T. barrerae, also are indicatative of genome doubling since the average 2C value of hystricognath rodents is 7.9 pg DNA (Gallardo et al., 2003). Increased nuclear size and cell dimensions result from quantum increases in genome size (Gregory and Hebert, 1999; Gregory, 2001). These nucleotypic effects are observed in the spermatozoa as well as in liver cells and lymphocytes of T. barrerae (Gallardo et al., 2002). They are also observed in the kidney mass of T. barrerae (1.27 ± 0.11g) which is double that of O. mimax (0.52 ± 0.11g; Díaz, 2001). The nucleus diameter of primordial and growing follicles as well as those of the Graafian follicles, of the granulose, and of luteal cells are significantly larger and heavily heterochromatic compared to S. cyanus (Gallardo et al., 2004b). This high heterochromatin content probably is associated with the doubling of the species genome so that redundant genetic information may be silenced through permanent chromatin modifications. The inclusion of nuclear and mitochondrial gene sequences for P. aureus did 14 University of California Publications in Zoology not alter the tree topology for the Octodontidae (Honeycutt et al., 2003) Fig. 2). Tympanoctomys and Pipanacoctomys are sister taxa, casting some doubts about the generic status of the latter.Octomys is sister to the Tympanoctomys-Pipanacoctomys clade, and forms a distinctive clade close to the root. The scansorial Octodontomys gliroides is basal to the second major clade which contains the Chilean genera Aconaemys, Octodon, and Spalacopus. The monophyletic derivation of Aconaemys and the genetic distinctiveness of its 3 species also is supported by chromosomal data (Gallardo and Mondaca, 2002). Likewise, the 3 Octodon species, recognized by DNA annealing data (Gallardo and Kirsch 2001), are corroborated by sequencing data (Honeycutt et al., 2003). Subterranean Spalacopus is related to fossorial Aconaemys, suggesting a common origin for adaptations to the underground niche (Fig. 2). No comparative genetic information exists for either the insular Octodon pacificus (Hutterer, 1994) or the deserticolous Salinoctomys loschalchalerosorum to test their taxonomic status and phylogenetic affinities. The extensive morphological, physiological, and ecological radiation of the depauperate octodontids contrasts with that of its highly speciose sister taxon, Ctenomys. This latter has radiated into approximately 60 species of similar bauplan, specialized in the exploitation of the subterranean niche only. These contrasting patterns of differentiation have been used to claim that extant Octodontidae are the remnants of an extensive past radiation (Reig, 1981). Nevertheless, standing diversities of Ctenomys and Octodontidae are not statistically different (Cook and Lessa, 1998). Molecular data and variation in DNA content implies that octodontid evolution was accompanied by a complex and saltational mode of chromosomal repatterning, probably through reticulate evolution. In contrast, genome size variation and the explosive chromosomal evolution in Ctenomys (2n = 10-70) may have followed a gradual pattern (Gallardo et al., 2003). The trajectories of genome evolution in these taxa are thus totally different and mark intrinsic differences, not considered by statistical tests. Genome duplication constitutes a novel mechanism of evolution in mammals and, as in plants, may be associated with quantum leaps to new adaptive zones without gradual interpopulational differentiation (Simpson, 1944).

ORIGIN OF GENOME DUPLICATIONS: A MODEL

Polyploidy in mammals is considered unlikely because the dosage compensation mechanism (X:A ratio) is disrupted after chromosome doubling (Orr, 1990). Subsequent imbalances derived from the constrained sex determination system further affect the normal developmental processes, and thus constitute an evolutionary dead end, as reported in humans (Goto and Monk, 1998). More recently, the lack of hybridization in animals has been argued to be the main factor preventing polyploidy in mammals (Otto and Whiton, 2000). However, since failure to hybridize is a consequence rather than a cause of genetic incompatibility, we consider the hybridization issue not to be a valid argument for the rarity of polyploid mammals. Phylogenetic proximity, strict bivalent formation at meiosis, matching combinations of chromosome numbers, and similar patterns of intergenomic southern hybridization among O. mimax, T. barrerae, and P. aureus support the notion of introgressive hybridization (Gallardo et al., 2004a). A hypothetical scenario to account for the origin of the duplicated genome of P. aureus (Fig. 3) assumes the production of non-reduced Gallardo et al.: The Octodontidae Revisited 15

Figure 2. Phylogenetic tree of the Octodontidae based on sequencing data of the mitochondrial 12S rDNA gene and the nuclear growth hormone receptor (GHR) gene. Abrocoma cinerea is used to root the tree. Diploid number for each species is given in parentheses. This total evidence tree is based on 1000 bootstrap iterations. gametes in 2 putative Octomys lineages that likely differed in chromosome number. Alternatively, tetraploidy in Pipanacoctomys could have originated by an additional set of maternal chromosomes (due to the incorporation of the polar body of meiosis I and double fertilization (dispermy) as reported in humans (Guc-Scekic et al., 2002; Baumer et al., 2003). Under this mechanism, tetraploid Pipanacoctomys males would inherit only one Y chromosome thereby avoiding the developmental failure experienced by double- Y tetraploid humans derived by endorreduplication (Goto and Monk, 1998). By the same token, the origin of T. barrerae may be hypothesized by assuming backcross of the tetraploid Pipanacoctomys lineage to the Octomys lineage. Again, either the production of unreduced gametes or double fertilization of a diploid oocyte is needed to account for the data and to explain the intriguing 102-chromosome complement of T. barrerae (Fig. 3). The recurrent production of unreduced gametes is a common feature in polyploid fish and plants (Pagliarini et al., 1999; Alves et al., 2001). On the other hand, if both parental species differed in chromosome number (dibasic origin) as inferred from the heteromorphic chromosome pairing observed in P. aureus, some chromosome combinations must have been selected against. These incompatibilities must have reduced the effective population size and the chances of successful matings. Thus, the frequency of of polyploid mammals seem to be constrained by the uncommon events of their genesis and subsequent lineage sorting for the karyotypic uniformity reported in T. barrerae and P. aureus. Nevertheless, further research will be needed to substantiate these ideas. Although the mechanisms to promote or initiate whole-genome duplications are unknown, the coincidental appearance of polyploids in extreme environments suggest the effect of external triggering factors. The Milankovitch climatic oscillation events in the Quaternary (e.g., dramatic environmental change in the past 1.8 Myr) have been advanced as factors promoting marked demographic and geographic 16 University of California Publications in Zoology

Figure 3. Proposed reticulated speciation events in the genesis of Pipanoctomys aureus and Tympanoctomys barrerae. Octomys “A” and “B” refer to putative species that probably no longer exist. The evolution of one of these lineages to originate extant O. mimax is suggested by dashed lines. The putative origin of Pipanacoctomys sp., is referred here as the “first” tetraploid taxon which subsequently evolved into extant P. aureus. changes. Although a single Milankovitch cycle (on a temporal scale of 104-105 yrs) is not necessarily implicated to cause such evolutionary changes, repeated cycles may have additive effects that do lead to such changes (Bennett 1990; Dynesius and Jansson, 2000; Lister, 2004). Moreover, the high percentage of polyploids among the halophytic flora on islands in the North Sea (65%) is thought to be causally associated to extreme saline habitats (Stebbins, 1971), although such a proposition is derived from the present distribution of these species, and not from original causes. It is also well known that the frequency of polyploid plants increases at higher latitude and altitude, and that the highest number of polyploids is found in areas covered with ice sheets during Pleistocene glacial maxima (Stebbins, 1971). For the octodontid rodents, the close timing between the formation of South American deserts and the origin of genome size duplication (6.5-7.0 mybp; Gallardo and Kirsch, 2001) may suggest a causal connection yet to be studied; divergence time between P. aureus and T. barrerae assuming a molecular clock indicates 2.8-3.1 myr whereas new data analysis indicates 1.8 myr, at the Pliocene-Pleistocene boundary (Opazo, 2005). The non-overlapping geographical ranges of P. aureus and T. barrerae have been interpreted as a result of vicariant events that broke the species geographic range Gallardo et al.: The Octodontidae Revisited 17 and allowed allopatric differentiation (Mares at al., 2000). Although the distributional range of these populations may have been broken during the Quaternary events of dry and humid cycles, the genetic barrier between them was achieved while in contact, as in plants with duplicated genomes (Stebbins, 1971). The co-occurrence of T. barrerae and O. mimax in Ischigualasto National Park (La Rioja, Argentina), and the proximity of P. aureus type locality to that area provides a plausible biogeographic setting for understanding the reticulate history proposed here for these species. A recent claim, using chromosome paints from Octodon degus hybridized to the mitotic plates of T. barrerae, concluded that the latter species is not in fact a tetraploid (Svartmann et al., 2005). The authors resort to discrediting arguments over tetraploidy by avoiding citation and discussion of recent molecular evidence (Gallardo et al., 2004a, 2004b). Moreover, their identification of the sex chromosomes of Octodon degus contradicts previous reports (Fernández, 1968; Gallardo 1992; Spotorno et al., 1995) but this discrepancy is not explained. In addition, the Y chromosome of T. barrerae and P. aureus (easily recognized as the only acrocentric chromosome in the male karyotype; Gallardo et al., 1999; Gallardo et al., 2004a) is misidentified as an autosome. This confusion stems from the assumption that gender could unequivocally be identified from a primary culture of a non-sexed T. barrerae embryo. We found their arguments against our work somewhat vague, especially because incorrect sex determination on their part resulted in a mis-paired karyotype (their Fig. 3B), which in turn was used as evidence to claim that large-scale heterochromatin accumulation explained the double genome size of the red vizcacha rat. Contrary to their conclusions, the chromosome mis-pairing of T. barrerae is not a consequence of its diploid condition, but a reflection of the species´ hybrid origin. Indeed, the banding heteromorphisms of T. barrerae strongly resemble the G- and C-banding differences in interspecific hybrids of the rodent genera Uromys (Baverstock et al., 1982) and Melomys (Baverstock et al., 1980). Methodologically, the use of chromosome painting has been criticized on several grounds when dealing with organisms having complex genomes intra- or interspecifically (Fuchs et al., 1996). The technique is highly inefficient at blocking disperse repetitive sequences (like in T. barrerae) and fails to detect the signal intensity of unique sequences. This may explain the paradox of obtaining stronger hybridization signals in heterologous assays relative to signal intensity in homologous hybridizations (Figs. 4B, 4C, 5B, 5C in Svartman et al., 2005). It seems likely that the stringency used to remove excessive background noise from the mitotic plates resulted in washing off weaker signals due to the limited affinity of heterologous probes to target chromosomes (Fuchs et al., 1996). Finally, we regret the authors´ confusion between quantum genome size shifts (Gallardo et al., 2003) and chromosome evolution in the octodontids. In fact, the 78-chromosome Octodon lunatus is half-way in a progression between 56 to102 chromosomes. Nevertheless, the 8.8 pg DNA of O. lunatus is not intermediate between diploid average in hystricognaths (7.9 pg DNA) and the record value of 16.8 pg DNA in T. barrerae, as argued by Svartmann et al. (2005).

THE SPECIAL CASE OF THE RED VIZCACHA RAT

The inflated bullae, specialized kidneys, and the rigid bundle of buccal hairs are part of the ecomorphological and physiological attributes that enabled the red vizcacha rat to cope with open, extreme habitats and their specialized food resources (Ojeda et al., 18 University of California Publications in Zoology

1996, 1999; Díaz and Ojeda, 1999; Díaz et al., 2000). Indeed, the restricted distributions of P. aureus and S. loschalchalerosorum (known from their type localities only; Mares et al., 2000) are in sharp contrast with that of T. barrerae (Fig. 1L-N). Fossil remains from the Atlantic coast of Argentina (Verzi et al., 2002) indicate a wider past distribution of the red vizcacha rat and imply that its present distributional record is a relict of a larger geographic range. The stratigraphic setting of this fossil of about 1myr, corresponding with the expansion of arid landscapes during the dry phases of the Quaternary events. The different distributional ranges between S. loschalchalerosorum, P. aureus, and T. barrerae argue against interspecific competition for ephemeral food items. Indeed, saltbush plants (Atriplex sp.) and other chenopodes constitute a stable food and water resource in most salt basins and sand dunes (Torres-Mura et al., 1989; Ojeda et al., 1996) such that the colonization success of T. barrerae has been advanced to result from its buccal modifications. This feature, the buccal comb unique to the red vizcacha rat, allows efficient peeling of Atriplex sp. leaves (Mares et al., 1997; Giannoni et al., 2000), which in association with other ecological, physiological, and behavioral attributes, enabled the species to occupy a new adaptive zone (Simpson, 1944).

Note added in proof

Recently, duplication of loci in multiple-copy (major rDNAs) and single-copy (Hoxc8) genes has been corroborated by fluorescence in situ hybridization in T. barrerae (Gallardo et al., 2006). Moreover, nucleolar dominance, a large-scale epigenetic silencing phenomenon characteristic of allopolyploids, has been demonstrated to explain the presence of one AG-NOR chromosome pair in the red vizcacha rat. Nucleolar dominance, together with the chromosomal heteromorphism detected in the G-banding pattern and synaptonemal complexes of the species´ diploid-like meiosis, consistently indicates allotetraploidy. Allotetraploidization can coherently explain the peculiarities of gene silencing, increased cell dimensions, and karyotypic evolution of T. barrerae that remain unexplained by assuming diploidy and a large genome size attained by the dispersion of repetitive sequences (Gallardo et al., 2006).

ACKNOWLEDGEMENTS

We are grateful to F. Mondaca, G. Díaz, M. Dacar, A. Ojeda and S. Tabeni, for collecting assistance, and to Benjamin Bender for preparing the maps. This work was partially funded by FNC 1010727 to MHG and CONICET PIP 2884 to RAO.

LITERATURE CITED

Alves, M. J., M. M. Coehlo, and M. J. Collares- Pereira 2001 Evolution in action through hybridization and polyploidy in an Iberian freshwater fish: a genetic review. Genetica 111:375-385. Gallardo et al.: The Octodontidae Revisited 19

Baumer A., D. Dres, S. Basaran, H. Isci, T. Dehgan, and A. Schinzel 2003 Parental origin of the two additional haploid sets of chromosomes in an embryo with tetraploidy. Cytogenetics and Genome Research 101:5-7.

Baverstock, P. R., M. Gelder, and A. Jahnke 1982 Cytogenetic studies of the Australian rodent Uromys caudimaculatus, a species showing extensive heterochromatin variation. Chromosoma 84:517-533.

Baverstock, P. R., C. H. S. Watts, M. Adams, and M. Gelder 1980 Chromosomal and electrophoretic studies of Australian Melomys (Rodentia: Muridae). Australian Journal of Zoology 28:553-574.

Bennett, K. D. 1990 Milankovitch cycles and their effects on species in ecological and evolutionary time. Paleobiology 16:11-21.

Contreras, L. C., J. C. Torres-Mura, and A. E. Spotorno 1990 The largest known chromosome number for a mammal in a South American desert rodent. Experientia 46:506-509.

Contreras, L. C., J. C. Torres-Mura, A. E. Spotorno, and F. M. Catzeflis 1993 Morphological variation of the glans penis of South American octodontid and abrocomid rodents. Journal of Mammalogy 74:926- 935.

Contreras, L. C., J. C. Torres-Mura, and J. L. Yáñez 1987 Biogeography of octodontid rodents: an eco-evolutionary hypothesis. Pp. 401-411 in Studies in Neotropical Mammalogy. Essays in Honor of P. Hershkovitz (Patterson,B. D., and R. E. Timm, eds.). Fieldiana: Zoology, new series 39.

Cook J. A., and E. Lessa 1998 Are rates of diversification in subterranean South American tuco-tucos (Genus Ctenomys, Rodentia, Octodontidae) unusually high? Evolution 52:1521-1527.

Díaz, G. B. 2001 Ecofisiología de Pequeños Mamíferos de las Tierras Áridas de Argentina: Adaptaciones Renales. Ph.D. Dissertation, Universidad Nacional de Cuyo, Mendoza, Argentina. 123 pp.

Díaz, G.B., and R. A. Ojeda 1999 Kidney structure of Argentine desert rodents. Journal of Arid Environments 41:453-461.

Díaz, G. B., R. A. Ojeda, M. H. Gallardo, and S. M. Giannoni 2000 Tympanoctomys barrerae. Mammalian Species 646:1-4. 20 University of California Publications in Zoology

Dynesius M., and R. Jansson 2000 Evolutionary consequences of changes in species´ geographic distributions driven by Milankovitch climate oscillations. Proceedings of the National Academy of Science (USA) 97:9115-9120.

Fernández, R. 1968 El cariotipo del Octodon degus (Rodentia-Octodontidae). Archivos de Biologia y Medicina Experimentales 5:33-37.

Fuchs, J., A. Houben, A., Brandes, and I. Schubert 1996 Chromosome “painting” in plants - a feasible technique? Chromosoma 104:315-320.

Gallardo, M. H. 1992 Karyotypic evolution in octodontid rodents based on C-band analysis. Journal of Mammalogy 73:89-98.

1997 A saltation model of karyotypic evolution in the Octodontoidea (Mammalia, Rodentia). Pp. 347-365 in Chromosomes Today, vol. 12 (Henríquez-Gil, N., J. S. Parker, and M. J. Puertas, eds.). Chapman and Hall, London, UK.

Gallardo, M. H. J. W. Bickham, G. Kausel, N. Köhler, and R. L. Honeycutt 1999 Discovery of tetraploidy in a mammal. Nature 401:341.

2003 Gradual and quantum genome size shifts in the hystricognath rodents. Journal of Evolutionary Biology 16:163-169.

Gallardo, M. H, O. Garrido, R. Bahamonde, and M. González 2004b Gametogenesis and nucleotypic effects in the tetraploid red vizcacha rat, Tympanoctomys barrerae (Rodentia, Octodontidae). Biological Research 37:767-775.

Gallardo, M. H., C. A. González, I. Cebrián. 2006 Molecular cytogenetics and allotetraploidy in the red vizcacha rat, Tympanoctomys barrerae (Rodentia, Octodontidae). Genomics, 88: 214- 221

Gallardo, M. H., G. Kausel, A. Jiménez, C. Bacquet, C. A. González, J. Figueroa, N. Köhler, and R. A. Ojeda 2004a Whole-genome duplications in South American desert rodents (Octodontidae). Biological Journal of the Linnean Society 82:443-451.

Gallardo, M. H., and J. A. W. Kirsch 2001 Molecular relationships among Octodontidae (Mammalia: Rodentia: Caviomorpha). Journal of Mammalian Evolution 8:73-89. Gallardo et al.: The Octodontidae Revisited 21

Gallardo, M. H., and F. Mondaca 2002 The systematics of Aconaemys (Rodentia, Octodontidae) and the distribution of A. sagei in Chile. Mammalian Biology 67:105-112.

Gallardo, M. H., F. Mondaca, R. A. Ojeda, N. Köhler, and O. Garrido 2002 Morphological diversity in the sperms of caviomorph rodents. Mastozoología Neotropical 9:159-170.

Giannoni, S. M., C. E. Borghi, and R. A. Ojeda. 2000 Foraging ecology of Tympanoctomys barrerae. Journal of Arid Environments 46:117-121.

Goto T, and M. Monk 1998 Regulation of X-chromosome inactivation in development in mice and humans. Microbiology and Molecular Biology Reviews 62:362-378.

Gregory, T. R. 2001 Coincidence, coevolution, or causation? DNA content, cell size, and the C-value enigma. Biological Review 76:65-101.

Gregory, T. R., and P. D. N. Hebert 1999 The modulation of DNA content: proximate causes and ultimate consequences. Genome Research 9:317-324.

Guc-Scekic, M., J. Milasin, M. Stevanovic, J. L. Stojanov, and M. Djorjevic 2002 Tetraploidy in a 26-month old girl (cytogenetic and molecular studies). Clinical Genetics 61:62-65

Honeycutt, R. L., D. L. Rowe, and M. H. Gallardo 2003 Molecular systematics of the South American caviomorph rodents: relationships among species and genera in the family Octodontidae. Molecular Phylogenetics and Evolution 26:476-489.

Hutterer, R. 1994 Island rodents: a new species of Octodon from Isla Mocha, Chile. Zeitschrift für Säugetierkunde 59:27-41.

Köhler, N., M. Gallardo, L. C. Contreras, and J. C. Torres-Mura 2000 Allozymic variation and systematic relationships of the Octodontidae and allied taxa (Mammalia, Rodentia). Journal of Zoology (London) 252:243-250.

Lister, A. M. 2004 The impact of Quaternary Ice Ages on mammalian evolution. Philosophical Transactions of the Royal Society of London B 359:221– 241. 22 University of California Publications in Zoology

Mares, M. A. 1985 Mammal faunas of xeric habitats and the Great American Interchange. Pp. 489-520 in The Great American Biotic Interchange (Stehli, F. G., and S. D. Webb, eds.). Plenum Publishing Corporation, New York, New York, USA.

Mares, M. A., J. K. Braun, R. M. Barquez, and M. M. Díaz 2000 Two new genera and species of halophytic desert mammal from isolated salt flats in Argentina. Occasional Papers, The Museum, Texas Tech University 203:1-27.

Mares, M. A., and R. A. Ojeda 1982 Patterns of diversity and adaptation in South American Histricognath Rodents. Pp. 393-432 in Mammalian Biology in South America (Mares, M. A., and H. H. Genoways, eds.). Pymatuning Laboratory of Ecology, Special Publication Series, 6. University of Pittsburgh, Pittsburgh, Pennsylvania, USA.

Mares, M. A., R. A. Ojeda, C. E. Borghi, S. M. Giannoni, G. B. Diaz, and J. K. Braun 1997 A desert rodent uses hair as a tool to overcome halophytic plant defenses. BioScience 47:699-704.

Muñoz-Pedreros, A. 2000 Orden Rodentia. Pp. 73-126 in Mamíferos de Chile (Muñoz Pedreros, A., and J. Yáñez-Valenzuela, eds). CEA Ediciones, Valdivia, Chile.

Nowak, R. M. 2001 Walker´s Mammals of the World, vol. II. Sixth edition. The Johns Hopkins University Press, Baltimore and London. 1936 pp.

Ojeda, R. A., C. E. Borghi, G. B. Diaz, S. M. Giannoni, M. A. Mares, and J. K. Braun 1999 Evolutionary convergence of the highly adapted desert rodent Tympanoctomys barrerae (Octodontidae). Journal of Arid Environments 41:443-452.

Ojeda, R. A., J. Gonnet, C. Borghi, S. Gianonni, C. Campos, and G. Diaz. 1996 Ecological observations of the red vizcacha rat Tympanoctomys barrerae in two desert habitats of Argentina. Mastozoología Neotropical 3:183-191.

Opazo, J. C. 2005 A molecular timescale for caviomorph rodents (Mammalia, Hystricognathi). Molecular Phylogenetics and Evolution 37:932-937.

Orr, H. A. 1990 “Why polyploidy is rarer in animals than in plants” revisited. American Naturalist 136:759-770. Gallardo et al.: The Octodontidae Revisited 23

Otto, S. P., and J. Whitton 2000 Polyploid incidence and evolution. Annual Reviews of Genetics 34:401- 437.

Oyarce, C. E, C. M. Campos, G. B. Díaz, and M. A. Dacar 2003 Hábitos alimentarios de Octomys mimax (Rodentia: Octodontidae), en el parque provincial Ischigualasto (San Juan, Argentina). Pp. 59, XVIII Jornadas SAREM, La Rioja, Argentina.

Pagliarini, M. S., S. Y. Takayama, P. M. de Freitas, L. R. Carraro, E. V. Adamowski, N. Silva, and L. A. R. Batista 1999 Failure of cytokinesis and 2n gamete formation in Brazilian accessions of Paspalum. Euphytica 108:129-135.

Patterson, B., and R. Pascual 1972 The fossil mammal fauna of South America. Pp. 247-309 in Evolution, Mammals and Southern Continents (Keast, A., F. C. Erk, and B. Glass, eds.). State University of New York Press, Albany, New York, USA.

Patterson, B., and A. E. Wood 1982 Rodents from the Deseadan Oligocene from Bolivia and the relationships of the Caviomorpha. Bulletin of the Museum of Comparative Zoology 149:371-543.

Redford, K. H., and J. F. Eisenberg 1992 Mammals of the Neotropics. Vol. 2, The Southern Cone: Chile, Argentina, Uruguay, Paraguay. The University of Chicago Press, Chicago, Illinois, USA. 430 pp.

Reig, O. 1981 Teoría del Origen y Desarrollo de la Fauna de Mamíferos de América del Sur. Monographie Naturae, Museo Municipal de Ciencias Naturales “Lorenzo Scaglia,” Mar del Plata, Argentina.

Simpson, B. B. 1975 Pleistocene changes in the flora of the high tropical Andes. Paleobiology 1:273-294.

Simpson, G. G. 1944 Tempo and Mode in Evolution. Columbia University Press, New York, New York, USA. 237 pp

Solbrig, O. 1976 The origin and floristic affinities of the South American temperate desert and semidesert regions. Pp. 7-49 in Evolution of Desert Biota (Goodall, D.W., ed.). University of Texas Press, Austin, Texas, USA. 24 University of California Publications in Zoology

Spotorno, A., L. Walker, L. C. Contreras, J. Pincheira, and R. Fernández Donoso 1988 Cromosomas ancestrales en Octodontidae y Abrocomidae. Archivos de Biologia y Medicina Experimentales 21:527.

Spotorno, A. E., L. Walter, L. C. Contreras, J. C. Torres-Mura, R. Fernández-Donoso, M. S. Berríos, and J. Pincheira 1995 Chromosome divergence of Octodon lunatus and the origins of Octodontoidea (Rodentia, Hystricognathi). Revista Chilena de Historia Natural 68:227-239.

Stebbins G. L. 1971 Chromosomal Evolution in Higher Plants. Edward Arnold Publishers, London, UK. 216 pp.

Svartman, M, G. Stone, and R. Stanyon 2005 Molecular cytogenetics discards polyploidy in mammals. Genomics 5:425.430.

Torres, M. R., C. A. Borghi, S. M. Giannoni, and A. Pattini 2003 Portal orientation and architecture of burrows in Tympanoctomys barrerae (Rodentia, Octodontidae). Journal of Mammalogy 84:541-546.

Torres-Mura, J. C., M. I. Lemus, and L. C. Contreras 1989 Herbivorous specialization of the South American desert rodent Tympanoctomys barrerae. Journal of Mammalogy 70:646-648.

Verzi, D. H. 1994 Origen y Evolución de los Ctenomyinae (Rodentia, Octodontidae): un análisis de anatomía cráneo-dentaria. Ph.D. Dissertation, Universidad Nacional de La Plata, Argentina. 226 pp.

2001 Phylogenetic position of Abalosia and the evolution of extant Octodontinae (Rodentia, Caviomorpha, Octodontidae. Acta Theriologica 46:243-268.

Verzi, D. H., E. P. Tonny, O. A. Scaglia, and J. O. San Cristobal 2002 The fossil record of the desert-adapted South American rodent Tympanoctomys (Rodentia, Octodontidae). Paleoenvironmental and biogeographic significance. Palaeogeography, Palaeoclimatology, Palaeoecology 179:149-158.

Webb, D. 1978 A history of savanna vertebrates in the New World, Part II. South America and the Great Interchange. Annual Reviews of Ecology and Systematics 9:393-426. Gallardo et al.: The Octodontidae Revisited 25

Wolkheimer, W. 1971 Aspectos paleoclimatológicos del Terciario Argentino. Revista del Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” e Instituto Nacional de Investigación de las Ciencias Naturales. Paleontología 1:243-262.

Wood A. E., and B. Patterson 1959 The rodents of the Deseadan Oligocene of Patagonia and the beginning of South American rodent evolution. Bulletin of the Museum of Comparative Zoology, Harvard University 120:279-428.

Woods, C. A. 1982 The history and classification of South American hystricognath rodents: Reflections on the far away and long ago. Pp. 377-392 in Mammalian Biology in South America (Mares, M. A., and H. H. Genoways, eds.). Pymatuning Laboratory of Ecology, Special Publication Series, 6. University of Pittsburgh, Pittsburgh, Pennsylvania, USA. 26 University of California Publications in Zoology