UVM Theses and Dissertations
Format:
Print
Author:
Axen, Heather Jylen
Dept./Program:
Biology
Year:
2011
Degree:
PhD
Abstract:
Hybridization has been considered an evolutionary dead end, but its evolutionary significance is becoming more widely recognized. Hybridization is associated with a reproductive tradeoff. Eusocial organisms, such as ants, characterized by a reproductive division of labor, may be able to overcome this tradeoff. Examples of hybridization in ants are common in the literature, but surprisingly few studies investigate its adaptive significance. I investigated the interplay between hybridization and eusociality in a fire ant hybrid zone between Solenopsis xyloni and S. geminata.
First, I investigated if mechanisms generating genetic differences between the worker and reproductive castes are environmentally or genetically based in colonies S. xyloni in the hybrid zone. I fostered queens collected from inside the hybrid zone and in allopatry with either pure species or hybrid workers, recording the proportion of queens that produced worker offspring. I found that the proportion of queens producing workers was not significantly different when queens with fostered with hybrid or S. xyloni workers; only offspring paternity predicted worker production. Solenopsis xyloni queens in the hybrid zone mated to their own species of male did not produce worker offspring in any context, suggesting that in the hybrid zone offspring caste determination is genetically fixed. This may represent the strongest case of genetic caste determination yet described, and it may have arisen because of conditions in the hybrid zone.
Second, I tested if hybrid workers were associated with advantages in the ecologically relevant traits of foraging and competitive ability. Colony fragments foraged for bait, and then colony pairs were allowed to compete for food and territory. The parental species, S. xyloni adapted to desert, and S. geminata adapted to savanna habitats, were more efficient at foraging and competition, respectively; hybrids were largely intermediate. This suggests that the parental species are specialized on opposite aspects of a discovery/dominance tradeoff, which may underlie the coexistence and exclusion of parents and hybrids inside and out of the hybrid zone. Solenopsis xyloni queens in the hybrid zone only occur with hybrid workers, suggesting the intermediate behavioral strategy of hybrids may be selectively advantageous in the hybrid zone.
Finally, I reconstructed phylogeographic relationships and evaluated genetic diversity in S. geminata to assess the timeframe of range expansions into the hybrid zone. I found that S. geminata is likely to have arisen in Mesoamerica, and appears to have had made several historical expansions from Mesoamerica into its current geographic distribution. One such expansion northward into what is presently the hybrid zone in Mexico and Texas, likely occurred 2.4-3.76 million years ago. This timeframe suggests that the hybrid zone may have persisteq over several million years, and the unique reproductive strategy found in S. xyloni may be evolutionarily stable.
Overall these studies suggest that eusocial organisms can reduce the reproductive costs associated with hybridization and may overcome the reproductive tradeoff; hybrids can be associated with non-reproductive advantages, suggesting hybridization may be important in evolutionary processes, and hybridization has resulted in a unusual form of caste determination in the hybrid zone, implying that understanding the interplay between hybridization and eusociality are important in understanding biological' complexity.
First, I investigated if mechanisms generating genetic differences between the worker and reproductive castes are environmentally or genetically based in colonies S. xyloni in the hybrid zone. I fostered queens collected from inside the hybrid zone and in allopatry with either pure species or hybrid workers, recording the proportion of queens that produced worker offspring. I found that the proportion of queens producing workers was not significantly different when queens with fostered with hybrid or S. xyloni workers; only offspring paternity predicted worker production. Solenopsis xyloni queens in the hybrid zone mated to their own species of male did not produce worker offspring in any context, suggesting that in the hybrid zone offspring caste determination is genetically fixed. This may represent the strongest case of genetic caste determination yet described, and it may have arisen because of conditions in the hybrid zone.
Second, I tested if hybrid workers were associated with advantages in the ecologically relevant traits of foraging and competitive ability. Colony fragments foraged for bait, and then colony pairs were allowed to compete for food and territory. The parental species, S. xyloni adapted to desert, and S. geminata adapted to savanna habitats, were more efficient at foraging and competition, respectively; hybrids were largely intermediate. This suggests that the parental species are specialized on opposite aspects of a discovery/dominance tradeoff, which may underlie the coexistence and exclusion of parents and hybrids inside and out of the hybrid zone. Solenopsis xyloni queens in the hybrid zone only occur with hybrid workers, suggesting the intermediate behavioral strategy of hybrids may be selectively advantageous in the hybrid zone.
Finally, I reconstructed phylogeographic relationships and evaluated genetic diversity in S. geminata to assess the timeframe of range expansions into the hybrid zone. I found that S. geminata is likely to have arisen in Mesoamerica, and appears to have had made several historical expansions from Mesoamerica into its current geographic distribution. One such expansion northward into what is presently the hybrid zone in Mexico and Texas, likely occurred 2.4-3.76 million years ago. This timeframe suggests that the hybrid zone may have persisteq over several million years, and the unique reproductive strategy found in S. xyloni may be evolutionarily stable.
Overall these studies suggest that eusocial organisms can reduce the reproductive costs associated with hybridization and may overcome the reproductive tradeoff; hybrids can be associated with non-reproductive advantages, suggesting hybridization may be important in evolutionary processes, and hybridization has resulted in a unusual form of caste determination in the hybrid zone, implying that understanding the interplay between hybridization and eusociality are important in understanding biological' complexity.