Yikweon Jang

Macroevolutionary Patterns in the Aphidini Aphids (Hemiptera: Aphididae): Diversification, Host Association, and Biogeographic Origins

Background Due to its biogeographic origins and rapid diversification, understanding the tribe Aphidini is key to understanding aphid evolution. Major questions about aphid evolution include origins of host alternation as well as age and patterns of diversification in relation to host plants. To address these questions, we reconstructed the phylogeny of the Aphidini which contains Aphis, the most diverse genus in the family. We used a combined dataset of one nuclear and four mitochondrial DNA regions. A molecular dating approach, calibrated with fossil records, was used to estimate divergence times of these taxa. Principal Findings Most generic divergences in Aphidini occurred in the Middle Tertiary, and species-level divergences occurred between the Middle and Late Tertiary. The ancestral state of host use for Aphidini was equivocal with respect to three states: monoecy on trees, heteroecy, and monoecy on grasses. The ancestral state of Rhopalosiphina likely included both heteroecy and monoecy, whereas that of Aphidina was most likely monoecy. The divergence times of aphid lineages at the generic or subgeneric levels are close to those of their primary hosts. The species-level divergences in aphids are consistent with the diversification of the secondary hosts, as a few examples suggest. The biogeographic origin of Aphidini as a whole was equivocal, but the major lineages within Aphidina likely separated into Nearctic, Western Palearctic, and Eastern Palearctic regions. Conclusions Most generic divergences in Aphidini occurred in the Middle Tertiary when primary hosts, mainly in the Rosaceae, were diverging, whereas species-level divergences were contemporaneous with diversification of the secondary hosts such as Poaceae in the Middle to Late Tertiary. Our results suggest that evolution of host alternation within Aphidini may have occurred during the Middle Tertiary (Oligocene) when the secondary hosts emerged.

Geographic Variation in Advertisement Calls in a Tree Frog Species: Gene Flow and Selection Hypotheses

Background In a species with a large distribution relative to its dispersal capacity, geographic variation in traits may be explained by gene flow, selection, or the combined effects of both. Studies of genetic diversity using neutral molecular markers show that patterns of isolation by distance (IBD) or barrier effect may be evident for geographic variation at the molecular level in amphibian species. However, selective factors such as habitat, predator, or interspecific interactions may be critical for geographic variation in sexual traits. We studied geographic variation in advertisement calls in the tree frog Hyla japonica to understand patterns of variation in these traits across Korea and provide clues about the underlying forces for variation. Methodology We recorded calls of H. japonica in three breeding seasons from 17 localities including localities in remote Jeju Island. Call characters analyzed were note repetition rate (NRR), note duration (ND), and dominant frequency (DF), along with snout-to-vent length. Results The findings of a barrier effect on DF and a longitudinal variation in NRR seemed to suggest that an open sea between the mainland and Jeju Island and mountain ranges dominated by the north-south Taebaek Mountains were related to geographic variation in call characters. Furthermore, there was a pattern of IBD in mitochondrial DNA sequences. However, no comparable pattern of IBD was found between geographic distance and call characters. We also failed to detect any effects of habitat or interspecific interaction on call characters. Conclusions Geographic variations in call characters as well as mitochondrial DNA sequences were largely stratified by geographic factors such as distance and barriers in Korean populations of H. japoinca. Although we did not detect effects of habitat or interspecific interaction, some other selective factors such as sexual selection might still be operating on call characters in conjunction with restricted gene flow.

Sex-Chromosome Homomorphy in Palearctic Tree Frogs Results from Both Turnovers and X–Y Recombination

Contrasting with birds and mammals, poikilothermic vertebrates often have homomorphic sex chromosomes, possibly resulting from high rates of sex-chromosome turnovers and/or occasional X-Y recombination. Strong support for the latter mechanism was provided by four species of European tree frogs, which inherited from a common ancestor (∼ 5 Ma) the same pair of homomorphic sex chromosomes (linkage group 1, LG1), harboring the candidate sex-determining gene Dmrt1. Here, we test sex linkage of LG1 across six additional species of the Eurasian Hyla radiation with divergence times ranging from 6 to 40 Ma. LG1 turns out to be sex linked in six of nine resolved cases. Mapping the patterns of sex linkage to the Hyla phylogeny reveals several transitions in sex-determination systems within the last 10 My, including one switch in heterogamety. Phylogenetic trees of DNA sequences along LG1 are consistent with occasional X-Y recombination in all species where LG1 is sex linked. These patterns argue against one of the main potential causes for turnovers, namely the accumulation of deleterious mutations on nonrecombining chromosomes. Sibship analyses show that LG1 recombination is strongly reduced in males from most species investigated, including some in which it is autosomal. Intrinsically low male recombination might facilitate the evolution of male heterogamety, and the presence of important genes from the sex-determination cascade might predispose LG1 to become a sex chromosome.

Colour and pattern change against visually heterogeneous backgrounds in the tree frog Hyla japonica.

Colour change in animals can be adaptive phenotypic plasticity in heterogeneous environments. Camouflage through background colour matching has been considered a primary force that drives the evolution of colour changing ability. However, the mechanism to which animals change their colour and patterns under visually heterogeneous backgrounds (i.e. consisting of more than one colour) has only been identified in limited taxa. Here, we investigated the colour change process of the Japanese tree frog (Hyla japonica) against patterned backgrounds and elucidated how the expression of dorsal patterns changes against various achromatic/chromatic backgrounds with/without patterns. Our main findings are i) frogs primarily responded to the achromatic differences in background, ii) their contrasting dorsal patterns were conditionally expressed dependent on the brightness of backgrounds, iii) against mixed coloured background, frogs adopted intermediate forms between two colours. Using predator (avian and snake) vision models, we determined that colour differences against different backgrounds yielded perceptible changes in dorsal colours. We also found substantial individual variation in colour changing ability and the levels of dorsal pattern expression between individuals. We discuss the possibility of correlational selection on colour changing ability and resting behaviour that maintains the high variation in colour changing ability within population.

Abiotic effects on calling phenology of three frog species in Korea

Abstract Calling behavior is often used to infer breeding patterns in anurans. We studied the seasonal and diel calling activities of anuran species in a wetland in central Korea to determine the calling season and to evaluate the effects of abiotic factors on male calling. Acoustic monitoring was used in which frog calls were recorded for a full day, once a week, throughout an entire year. Using acoustic monitoring, we identified three frog species in the study site. Males of Rana dybowskii called in late winter and early spring; we thus classified this species as a winter/spring caller. The results of binary logistic regression showed that temperature, relative humidity, and 1-day lag rainfall were significant factors for male calling in R. dybowskii. Temperature and relative humidity were important factors for the calling activity of R. nigromaculata, whereas 24-h rainfall and 1-day lag rainfall were not significant. Thus, we determined R. nigromaculata to be a summer caller independent of weather. In Hyla japonica, relative humidity, 24-h rainfall, and 1-day lag rainfall were significant for male calling, suggesting that this species is a summer caller dependent on local rain.

Spatiotemporal distributions and habitat characteristics of the endangered treefrog, Hyla suweonensis, in relation to sympatric H. japonica

Abstract A comparison between localities currently occupied by a species and localities where the species recently underwent local extinction is a powerful way to understand the causes of population decline. Two closely related treefrog species occur in the Republic of Korea: Hyla japonica , which is distributed widely throughout northeastern Asia, and H. suweonensis , which occurs sympatrically on a narrow strip of the western coastal plains in Korea. Males of both species produce species-specific advertisement calls. To determine the spatiotemporal distributions of H. suweonensis and H. japonica during their breeding seasons, auditory monitoring was conducted in 70 localities throughout the historic range of H. suweonensis . A group of volunteers conducted the monitoring, supplemented with call recordings for validation. The endangered H. suweonensis was found in only 14 of 70 localities, designated as syntopic, whereas H. japonica occurred in all 70 localities. The calling activity of H. japonica was constantly vigorous throughout the study period, whereas the calling activity of H. suweonensis was generally weak. The analysis of binary logistic regression identified the percentage of rice-paddy field as the significant variable. The syntopic localities had much higher percentages of the rice-paddy field than the non-syntopic localities did. Localities where the rice-paddy fields are fragmented and isolated by commercial and residential complexes are not likely to harbor the H. suweonensis populations. The spatiotemporal distributions of the two treefrog species throughout the breeding seasons confirm that H. suweonensis is indeed rare and requires swift and decisive measures, including the establishment of protected areas.

No reproductive character displacement in male advertisement signals of Hyla japonica in relation to the sympatric H. suweonensis

Reproductive interaction between closely related taxa may leave a distinctive signature in which populations of interacting taxa are more dissimilar in sympatry than in allopatry. An ideal condition for such a pattern of reproductive character displacement (RCD) may occur when a population has limited gene flow and experiences strong selection pressure, exerted by an interacting taxon in areas of sympatry. In Korea, there are two closely related treefrog species: Hyla japonica, which is distributed widely throughout the country, and Hyla suweonensis, which occurs sympatrically in a narrow strip of western coastal plains. H. suweonensis is only found within the distribution of H. japonica. These two species have a similar single-note call structure. Here, we tested the possibility of RCD in H. japonica by examining geographic variation in advertisement calls. Although means of temporal and spectral characters were significantly different between the two species, sympatric populations of H. japonica and H. suweonensis overlapped in distributions of most characters. Furthermore, allopatric and sympatric H. japonica populations did not differ in all call characters. Weak genetic differentiation between sympatric and allopatric populations of H. japonica implied either substantial gene flow or recent genetic isolation. Possible explanations for no RCD in male advertisement calls of H. japonica include a difference in fine temporal characteristics between the two species, migration between sympatric and allopatric localities in H. japonica, RCD in female preferences in H. japonica, and weak selection pressure by H. suweonensis.

Description of a seminatural habitat of the endangered Suweon treefrog Hyla suweonensis

Amphibian populations have been declining worldwide over the last few decades due to causes like habitat modification, climate change, and emergent diseases. Hyla suweonensis, a treefrog species endemic to Korea, has disappeared from much of its historic distributional range. Past studies reported this species to breed exclusively in man-made rice paddies. This study reports the habitat characteristics of a seminatural site for H. suweonensis. Out of 100 sites where H. suweonensis was present, only one was not a rice paddy. This seminatural site was characterized by a network of swamps bordered with forests of willow trees. H. suweonensis individuals located on willow trees called from 0.2 to 1.50 m above ground at the edge of the forest near the swamps. The seminatural site has a large percentage of rice paddies with a very low percentage of urban areas, a feature consistent with findings of a previous study. H. suweonensis was found calling with two other amphibian species. Although extracted from a single site, the information from this study is important for the management and the conservation of the species.

Convergent and divergent patterns of morphological differentiation provide more evidence for reproductive character displacement in a wood cricket Gryllus fultoni(Orthoptera: Gryllidae)

BackgroundIn ecological character displacement, traits involved in reproductive isolation may not evolve in arbitrary directions when changes in these traits are by-products of adaptation to an ecological niche. In reproductive character displacement, however, selection acts directly on reproductive characters to enhance the degree of reproductive isolation between sympatric populations. Thus, the direction of change in reproductive characters may be arbitrary in relation to changes in other morphological characters. We characterized both tegminal characters and characters indicative of body size in sympatric and allopatric populations of Gryllus fultoni, a species displaying character displacement in its calling song characters in areas of sympatry with G. vernalis populations, to infer the nature and direction of selection acting on reproductive and morphological characters in sympatry.ResultsExcept for mirror area, the number of teeth in a file, and ovipositor length of G. fultoni, all male and female morphological characters in G. fultoni and G. vernalis exhibited a uniform tendency to decrease in size with increasing latitude. There was no significant variation in female morphological characters between sympatric and allopatric G. fultoni populations. However, males of sympatric and allopatric G. fultoni populations significantly differed in head width, hind femur length, and mirror area even after controlling for clinal factors. Head width and hind femur length of G. fultoni were more similar to those of G. vernalis in sympatric populations than in allopatric populations, resulting in morphological convergence of G. fultoni and G. vernalis in sympatry. However, the mirror area of G. fultoni displayed the divergent pattern in relation to the sympatric G. vernalis populations.ConclusionDivergence-enhancing selection may be acting on mirror area as well as calling song characters, whereas local adaptation or clinal effects may explain variation in other morphological characters in sympatric populations of G. fultoni. This study also suggests that structures and behaviors that directly enhance reproductive isolation may evolve together, independently of other morphological traits.

Temporal and spatial differentiation in microhabitat use: Implications for reproductive isolation and ecological niche specification

Niche differentiation enables ecologically similar species to coexist by lessening competition over food and/or shelters and may be critical for reproductive isolation between closely related species in close proximity. Because no extra traits need to evolve, spatial and temporal differentiation may readily take place to complement other isolating mechanisms. Two closely related treefrog species occur together in Korea: the endangered Hyla suweonensis and the widespread Hyla japonica. Advertisement calls are differentiated, but it is unclear whether call difference is sufficient for reproductive isolation. We tracked individuals of both species to study fine-scale differentiation in microhabitat use in the diel cycle of the breeding season using a harmonic direction finder. tracking male movement patterns of both species revealed spatial and temporal differentiation in microhabitat use for calling and resting during the breeding season. Males of both H. suweonensis and H. japonica occurred in all 5 microhabitats identified in this study: rice paddy, ground, buried, grass and bush. Both treefrog species showed general similarities in calling from rice paddies and resting in grass and bush. However, H. suweonensis moved into rice paddies and produced advertisement calls 3 h earlier than H. japonica. These differences likely minimize contact between the species and provide an additional isolating mechanism. In addition, the activity of H. suweonensis may be contributing to the decline of this species, as resting in grass would increase dangers from predatory birds and habitat disturbance.