Hot and Cold spots of selection: Parasitoid attack of fungus-growing ants
Abstract: It is known that fungus-growing ants, and their integral mutualism with fungus, allowed for their exploitation of new niches; which resulted in diversified lineages. However, there may have been other factors that refined these lineages into the resulting clades and species we see today. Diapriidae wasps are extremely common in the Neotropics; and poorly understood phylogenetically and ecologically. However, Diapriidae parasitoids in the same genera are known to parasitize both Cyphomyrmex and Trachymyrmex (Formicidae: Attini). The parasite wedge theory suggests speciation occurred in parapatric populations from parasitoid pressures. This theory is rooted in the geographic mosaic theory of coevolution, which has shown that populations of species interacting with other species, can lead to the coevolution of the interacting species. Using these theories, it may be possible to explain the finer branches of fungus-growing ant evolution, while revealing new information about a multi-host-parasitoid system. Geographic Mosaic Theory of Coevolution and Parasite Wedge Theory The geographic mosaic theory suggests that a spectrum of individuals in a population are interacting, intra- and interspecifically, which results in coevolution. There are different strengths of selective pressures which influence coevolution between species interactions; through predator-prey, host-parasitoid, and mutualist interactions 1. Components required for the geographic mosaic theory are: evolutionary relationships, specialization on other species, variation in the species interactions, variability in the rate of selection within populations, and their environment 1. When these occur they result in “hot” or “cold” spots of selection within and between populations, that occur along clines at large and small scales 1,2. Because of the geographic mosaic, hot and cold spots mix and prevent gene flow, random genetic drift occurs, and coevolution results between the interacting species. This theory is useful in understanding, and helps develop, the parasite wedge theory. The parasite wedge theory predicts speciation in parapatric populations 3. …show more content…
In this theory, parasites act as selective pressures. Populations of host-parasites are patchy, and unevenly distributed as in the geographic mosaic theory 3,4. As hosts and parasites interact, they adapt in response to each other in an evolutionary arms race 5. Populations with higher proportions of parasites may have reduced dispersal and gene flow compared to populations with low parasite pressure 3,4. Alternatively, individuals with unique alleles or behaviors will arise that would be adaptive against the parasites, or provide a means for escape from them. Over time, these alleles or behaviors of parasite escape accrue and eventually lead to reproductive isolation 3,4. When parts of a host-parasite population diverge from their ancestral population, cryptic species can emerge parapatricaly 6. There are good examples of cryptic species, coevolution 7, and parasitism found in Fungus-Growing ants in the tribe Attini (Hymenoptera: Formicidae). Table 1 - Species of fungus-growing ants described since the year 2000, data from antweb.org. Genera New species Acromyrmex 1 Apterostigma 2 Cyatta 1 Cyphomyrmex 5 Mycetagroicus 4 Mycocepurus 1 Myrmicocrypta 3 Trachymyrmex 5 Total…………………....…….20 Fungus-Growing Ants and their Parasitoids Fungus-growing ants are ubiquitous organisms throughout much of the new world. The monophyletic group of around ca. 250 species, appeared nearly 60 million years ago 8-10. Attines are an ecologically important group that harvest a wide range of vegetation from their immediate environment, which they feed to their mutualist fungus 10,11. Because of their complex fungus-growing symbiosis, fungus-growing ants have radiated and diversified successfully throughout the new world. New species of these unique fungus-growing ants continue to be discovered and described to this day (e.g. – 12,13), see Table 1. The fungus-growing ant species Cyphomyrmex muelleri, was found to be a separate from its sister species Cy. longiscapus, described in the early 2000’s 13. More recently Trachymyrmex fovater sp. n. was also separated from its sister species T. zeteki as well 12,14,15. The sister Trachy. species are found mostly near creek banks, but have also been found in forests as well 14. Both genera of sister species belong to sister clades within Attini 11, both genera and species occur sympatrically 13, and they nest in similar habitats near creeks on vertical embankments with unique nest entrances 13,14. Because of the Attines natural diversity, they represent a unique resource for other organisms, which attracts a
Abstract: It is known that fungus-growing ants, and their integral mutualism with fungus, allowed for their exploitation of new niches; which resulted in diversified lineages. However, there may have been other factors that refined these lineages into the resulting clades and species we see today. Diapriidae wasps are extremely common in the Neotropics; and poorly understood phylogenetically and ecologically. However, Diapriidae parasitoids in the same genera are known to parasitize both Cyphomyrmex and Trachymyrmex (Formicidae: Attini). The parasite wedge theory suggests speciation occurred in parapatric populations from parasitoid pressures. This theory is rooted in the geographic mosaic theory of coevolution, which has shown that populations of species interacting with other species, can lead to the coevolution of the interacting species. Using these theories, it may be possible to explain the finer branches of fungus-growing ant evolution, while revealing new information about a multi-host-parasitoid system. Geographic Mosaic Theory of Coevolution and Parasite Wedge Theory The geographic mosaic theory suggests that a spectrum of individuals in a population are interacting, intra- and interspecifically, which results in coevolution. There are different strengths of selective pressures which influence coevolution between species interactions; through predator-prey, host-parasitoid, and mutualist interactions 1. Components required for the geographic mosaic theory are: evolutionary relationships, specialization on other species, variation in the species interactions, variability in the rate of selection within populations, and their environment 1. When these occur they result in “hot” or “cold” spots of selection within and between populations, that occur along clines at large and small scales 1,2. Because of the geographic mosaic, hot and cold spots mix and prevent gene flow, random genetic drift occurs, and coevolution results between the interacting species. This theory is useful in understanding, and helps develop, the parasite wedge theory. The parasite wedge theory predicts speciation in parapatric populations 3. …show more content…
In this theory, parasites act as selective pressures. Populations of host-parasites are patchy, and unevenly distributed as in the geographic mosaic theory 3,4. As hosts and parasites interact, they adapt in response to each other in an evolutionary arms race 5. Populations with higher proportions of parasites may have reduced dispersal and gene flow compared to populations with low parasite pressure 3,4. Alternatively, individuals with unique alleles or behaviors will arise that would be adaptive against the parasites, or provide a means for escape from them. Over time, these alleles or behaviors of parasite escape accrue and eventually lead to reproductive isolation 3,4. When parts of a host-parasite population diverge from their ancestral population, cryptic species can emerge parapatricaly 6. There are good examples of cryptic species, coevolution 7, and parasitism found in Fungus-Growing ants in the tribe Attini (Hymenoptera: Formicidae). Table 1 - Species of fungus-growing ants described since the year 2000, data from antweb.org. Genera New species Acromyrmex 1 Apterostigma 2 Cyatta 1 Cyphomyrmex 5 Mycetagroicus 4 Mycocepurus 1 Myrmicocrypta 3 Trachymyrmex 5 Total…………………....…….20 Fungus-Growing Ants and their Parasitoids Fungus-growing ants are ubiquitous organisms throughout much of the new world. The monophyletic group of around ca. 250 species, appeared nearly 60 million years ago 8-10. Attines are an ecologically important group that harvest a wide range of vegetation from their immediate environment, which they feed to their mutualist fungus 10,11. Because of their complex fungus-growing symbiosis, fungus-growing ants have radiated and diversified successfully throughout the new world. New species of these unique fungus-growing ants continue to be discovered and described to this day (e.g. – 12,13), see Table 1. The fungus-growing ant species Cyphomyrmex muelleri, was found to be a separate from its sister species Cy. longiscapus, described in the early 2000’s 13. More recently Trachymyrmex fovater sp. n. was also separated from its sister species T. zeteki as well 12,14,15. The sister Trachy. species are found mostly near creek banks, but have also been found in forests as well 14. Both genera of sister species belong to sister clades within Attini 11, both genera and species occur sympatrically 13, and they nest in similar habitats near creeks on vertical embankments with unique nest entrances 13,14. Because of the Attines natural diversity, they represent a unique resource for other organisms, which attracts a