Patterns of evolution inGraomys griseoflavus (Rodentia, Muridae). III. Olfactory discrimination as a premating isolation mechanism between cytotypes

Abstract

Graomys griseoflavus is a South American murid rodent showing a high degree of chromosomal polymorphism. We have recently demonstrated that cytotypes 2n = 36-38 form a complex reproductively isolated from the kariomorph 2n = 42, and that they may be considered separate species. Given the importance of olfactory perception for precopulatory recognition, we have investigated the ability of G. griseoflavus females to distinguish between odiferous stimuli from conspecific and heterospecific males by means of olfactory discrimination tests. Results show that females are able to recognize olfactory stimuli from males of compatible chromosome complement and those from males with whom they would not produce viable descendants or would produce sterile hybrids. This discrimination ability is only evident during the estrus phase, suggesting that this capacity is related to the precopulatory recognition and could act as a premating reproductive isolation mechanism. o 1996 Wiley-Liss, Inc.The South American murid rodent Graomys griseoflavus shows a high degree of chromosomal polymorphism (Ortells et al., '89; Zambelli et al., '94). Chromosome complements 2n = 36, 37, 38, and 42 have been found in individuals which so far had been considered members of a single species (Wainberg and Gentile de Fronza, '74; Pearson and Patton, '76; Zambelli et al., '94). Different karyomorphs are morphologically indistinguishable. In Argentina citotypes 2n = 36, 37, and 38 cohabit in the phytogeographic area known as "monte." Although individuals 2n = 42 are found preferentially in the phytogeographic region designated "espinal'' (Fig. l), their distribution area overlaps with that of 2n = 36-38 in borderlands.Allozymic analysis of different populations of Graomys griseoflavus showed that the cytotypes 2n = 42 and those 2n = 36-38 have undergone almost no divergence from the point of view of structural genes (Theiler and Gardenal, '94). Ge- This is an example of speciation through sequential chromosomal rearrangements.It has been observed that the 2n = 36, 37, and 38 cytotypes are interfertile, belonging to a single species complex (Theiler and Blanco, '96).Crosses performed in the laboratory between 2n = 42 females and 2n = 36-38 males were unproductive, while 95% of the reciprocal crosses produced viable but sterile hybrids. These hybrids, with 2n = 40 or 39, could not be found in field populations in regions where the distribution areas of the species 2n = 42 and 2n = 36-38 overlap (Theiler and Blanco, '96). This suggests that hybrids are not produced or are very rare in natural conditions. Thus, it is possible that some mechanism of premating reproductive isolation prevents interspecific crosses.Given the importance of the olfactory perception for precopulatory recognition between members of the same and related species (Bowers and Alexander, '67; Doty, '73; Moore, '65; Nevo et al., '76; Nevo and Heth, '76; Parkes and Bruce, '61; Smith, '65), we have investigated the ability of

Autosomal rearrangements in Graomys griseoflavus (Rodentia): a model of non-random Robertsonian divergence

“…In the distribution area 2n=42–41 individuals inhabit the “Espinal” and “Western Chaco” phytogeographic regions located in central Argentina, while animals of complex 2n=38, 37, 36, 35 and 34 (2n=38–34) are found mainly at “Monte” region, in the western‐central area of the country. According to T heiler and B lanco (1996a) and T iranti (1998) there are no significant geographic barriers separating different populations of Graomys; in fact, these authors described narrow overlapping zones in some regions. Based on the cytogenetic and geographical distribution features, one could presume that Graomys agrees with the sympatric chromosomal speciation model proposed by W hite (1978).…”

“…T heiler and B lanco (1996a, 1996b) also analysed the reproductive behavior of G. griseoflavus , finding interfertility among 2n=36, 37, and 38 karyotypes through laboratory mating tests. Those studies also showed the existence of reproductive isolation between 2n=36–38 complex and 2n=42 karyomorph: crosses between 2n=36–38 males and 2n=42 females were unreproductive, while crosses between 2n=42 males and 2n=36–38 females produced hybrid descendants.…”

“…Those studies also showed the existence of reproductive isolation between 2n=36–38 complex and 2n=42 karyomorph: crosses between 2n=36–38 males and 2n=42 females were unreproductive, while crosses between 2n=42 males and 2n=36–38 females produced hybrid descendants. All hybrid males and 80% hybrid females were sterile, while the remaining 20% showed very low fertility in backcrosses (T heiler and B lanco 1996a).…”

Morphometrics of Graomys (Rodentia, Cricetidae) from Central-Western Argentina

“…The reproductive isolation was clearly asymmetric; there was no copula between males 2n ¼ 36-38 and females 2n ¼ 42, whereas the reciprocal crossings (females 2n ¼ 36-38 Â males 2n ¼ 42) produced viable sterile hybrid offspring with 39 or 40 chromosomes (Theiler and Blanco 1996a). Theiler and Blanco (1996b) also found reinforcement of this post-zygotic isolation by mechanisms involving olfactory discrimination during female estrus. Females discriminated males of compatible chromosome complement from those that would produce non-viable offspring or sterile hybrids.…”

“…Females discriminated males of compatible chromosome complement from those that would produce non-viable offspring or sterile hybrids. On the basis of these evidences, Theiler and Blanco (1996b) concluded that 2n ¼ 42 cytotype and 2n ¼ 36-38 complex are two sibling species.…”