Why are there so few hybrids in the Green-eyed Treefrog hybrid zone? Resolving the role of behaviour versus genetics

A project undertaken at the School of Marine and Tropical Biology, James Cook University, and supervised by Dr Conrad Hoskin & Dr Megan Higgie

The question of how species are formed (speciation) remains an integral question in evolutionary biology. Particularly topical is the plausibility and importance of the process of ‘reinforcement’, in which premating isolation and speciation can evolve between two lineages due to natural selection against maladaptive hybridization. The Green-eyed Treefrog (Litoria serrata) is a rainforest frog restricted to the Wet Tropics region of north-east Queensland. It consists of two deeply divergent lineages (northern and southern) that meet in contact zones in the central Wet Tropics. Reinforcement has led to speciation between the two lineages at one hybrid zone (to form Litoria myola, Hoskin 2007), but apparently not at an independent hybrid zone approximately 12 km to the south in Lamb Range (Hoskin et al. 2005). Over the last few years we have been studying these hybrid zones in detail to address the question of why reinforcement occurred at one hybrid zone but apparently not at the other. Interestingly, we have found that there are very few hybrids at both contact zones. This was expected for the hybrid zone where reinforcement has occurred (where premating isolation is strong) but the rarity of hybrids at the other hybrid zone is surprising. We will use the funding from the Australia and Pacific Science Foundation to work out what limits hybridization between the two lineages in this contact zone.

The two possibilities we will test are:

1. Female mate choice has been reinforced in the hybrid zone such that they correctly choose the calls of males of their own lineage (i.e. premating isolation limits hybridization).
2. Females do not display strong choice for their own lineage but few hybrid offspring survive to adulthood due to substantial genetic incompatibility between the two lineages (i.e. postmating isolation).

We will study mate choice and hybrid tadpole frequency in the hybrid zones to test between these two alternatives. The results will contribute towards understanding the processes that determine whether reinforcement occurs in hybrid zones and the importance of hybrid zones in generating diversity.

References

Hoskin CJ, Higgie M, McDonald KR & Moritz C. 2005. Reinforcement drives rapid allopatric speciation. Nature 437: 1353-1356.

Hoskin CJ. 2007. Description, biology and conservation of a new species of Australian tree frog (Anura: Hylidae: Litoria) and an assessment of the remaining populations of Litoria genimaculata Horst, 1883: systematic and conservation implications of an unusual speciation event. Biological Journal of the Linnean Society 91: 549-563.