When hybrids are best
This week's paper is Facultative Mate Choice Drives Adaptive Hybridization by Karin Pfennig of the University of North Carolina, Chapel Hill (where I went to kindergarten), published in Science.
Spadefoot Toad (Wikipedia)
In contrast with the â€śhybrid vigorâ€? sometimes seen with crosses between different genotypes within a species -- corn, for example -- hybrids resulting from mating between related species are usually less likely to survive and reproduce. For example, when two different species of spadefoot toad mate, their daughters usually produce fewer eggs. But apparently there are situations where genes from the other species are beneficial enough to outweigh problems due to genetic incompatibilities.
When Spea bombifrons females mate with Spea multiplicata males, their tadpoles mature faster. This is beneficial in shallow ponds that donâ€™t last long, but not in deeper ponds, which will last long enough for nonhybrid tadpoles to mature. Therefore, the smart thing for S. bombifrons females to do would be to mate with their own species in deeper ponds, but with S. multiplicata in shallower ponds. But how smart is the average toad?
To find out, Dr. Pfennig looked at the response of S. bombifrons females to recorded calls of S. bombifrons males Download file (their own species) versus S. multiplicata males Download file, to see how they respond. (Mark Bee, in my department, uses similar methods.) In deep pools, they moved towards the call of their own species about 2/3 of the time, but in shallow pools they were slightly more likely to approach the call of an S. multiplicata male. There was some evidence of individual preference also, with each female choosing the same species call 76% of the time. Toads in worse physical condition were slightly more likely to switch species, consistent with the observation that tadpoles from healthier mothers mature faster, so are less at risk in rapidly drying shallow ponds.
What controls these preferences? There are at least two aspects to consider. First, the ability to distinguish the calls of different species is needed for any kind of preference. This ability apparently evolves only when both species are present, as S. bombifrons females from areas where S. multiplicata is not found often chose the other species, even in deep-water trials. This was a little surprising to me after I got to hear how different the calls sound -- thanks to Karin Pfennig for sending them -- go back and download the audio files if you skipped them. But female preferences can apparently evolve fast enough to have changed since they migrated to (or from) areas where S. multiplicata is found.
This seems to contrast with data presented in a recent seminar here. Michael Ryan showed that, in the tungara frogs he studies, female preferences have been inherited from their fairly distant ancestors, while the â€śtuningâ€? of male calls to match those preferences came later. (Hungry bats that home in those calls make mating a dangerous game.)
This is another example of the sophistication of reproductive strategies in species with very small brains. For example, female wasps that pollinate figs adjust the sex ratio of their offspring, depending on how many other females are laying eggs in the same fruit. When only one wasp is present, she lays mostly female eggs; all her sons will die in the fruit after mating with their sisters, and only daughters will carry her genes on. But if other wasps are present, her sons can carry on her genes by mating with another waspâ€™s daughters, so she lays more male eggs. Recently, it was found that this sex ratio adjustment is even better than we thought. When two different â€ścryptic speciesâ€? are present, we canâ€™t tell them apart (except by their DNA), but apparently the wasps can, and adjust the sex ratio of their eggs accordingly (PNAS 100:5867). The wasps die after laying their eggs, so we could be sure this behavior is evolved rather than learned, even if their brains were bigger.
Also this week: