Biology subjects

McMillan, W. O.

Publications and source records attributed to McMillan, W. O..

6 recordsLinked to original sources

bioRxiv2018

Experimental manipulation of Heliconius warning patterns reduces harassment of previously mated females.

Why warning patterns are so diverse is an enduring evolutionary problem. Because predators learn to associate particular patterns with unpleasant experiences, an individuals risk of predation should decrease as the local density of its warning pattern increases. Heliconius butterflies, however, are known for their diversity of warning patterns, and the establishment of entirely new phenotypes is difficult to explain under strict frequency-dependent selection. One possibility is that during periods of relaxed selection, drift may allow new variants to rise above a threshold density until mimicry selection takes over. We propose an alternative hypothesis where novel pattern phenotypes arise due to a conflict of interests between the sexes. It is well established that male Heliconius use warning patterns as a mating cue. This will likely be beneficial to males as it will increase the efficiency of finding mates. However, already mated females may suffer fitness costs if these cues lead to harassment by males during oviposition or foraging. When constraints imposed by predation are locally relaxed, this could lead to rapid divergence in pattern phenotypes through chase-away sexual selection. To begin to test this hypothesis, we experimentally manipulated the warning patterns of mated Heliconius erato demaphoon females and recorded their interactions with conspecific males, and the effect of male presence on laying rate. As predicted, males interacted less with mated females whose red forewing band was blacked-out, as compared to control females whose warning pattern remained intact. We also show that females lay less eggs in the presence of males, but we were unable to detect a significant interaction between warning pattern treatment and the presence of males on female fecundity. Our results suggest that male attraction to conspecific warning patterns, may impose a previously unrecognized cost on Heliconius females.

evolutionary biology↗

bioRxiv2018

Male pheromone composition depends on larval but not adult diet in Heliconius melpomene

Condition-dependent traits can act as honest signals of mate quality, with fitter individuals able to display preferred phenotypes. Nutrition is known to be an important determinant of individual condition, with diet known to affect many secondary sexual traits. In Heliconius butterflies, male chemical signalling plays an important role in female mate choice. Heliconius pheromones are sexually dimorphic, found only in mature males, but it is unclear what information they convey to the female. Here, we manipulate both the larval and adult diet of male Heliconius melpomene rosina to test for environmental effects on wing and genital pheromone production. We find no evidence that adult pollen consumption affects pheromone production in the first ten days after eclosion. We also find strong overlap in the chemical profiles of individuals reared on different larval host plants. However, individual compounds were found in different amounts between host plant treatments. Further electrophysiological and behavioural experiments will be needed to determine the biological significance of these differences.

ecology↗

bioRxiv2018

Genetic dissection of assortative mating behavior

The evolution of new species is made easier when traits under divergent ecological selection are also mating cues. Such ecological mating cues are now considered more common than previously thought, but we still know little about the genetic changes underlying their evolution, or more generally about the genetic basis for assortative mating behaviors. The warning patterns of Heliconius melpomene and H. cydno are under disruptive selection due to increased predation of non-mimetic hybrids, and are used during mate recognition. We carried out a genome-wide quantitative trait locus (QTL) analysis of preference behaviors between these species and showed that divergent male preference has a simple genetic basis. Three QTLs each explain a large proportion of the differences in preference behavior observed between the parental species. Two of these QTLs are on chromosomes with major color pattern genes, including one that is tightly associated with the gene optix. Different loci influence different aspects of attraction, suggesting that behavioral isolation in Heliconius involves the evolution of independently segregating modules, similar to those for the corresponding wing pattern cues. Hybridization and subsequent sharing of wing pattern loci has played an important role during adaptation and speciation in Heliconius butterflies. The existence of large effect preference loci could similarly assist the evolution of novel behavioral phenotypes through recombination and introgression, and should facilitate rapid speciation.

evolutionary biology↗

bioRxiv2017

Patterns of Z chromosome divergence among Heliconius species highlight the importance of historical demography

Sex chromosomes are disproportionately involved in reproductive isolation and adaptation. In support of such a large-X effect, genome scans between recently diverged populations or species pairs often identify distinct patterns of divergence on the sex chromosome compared to autosomes. When measures of divergence between populations are higher on the sex chromosome compared to autosomes, such patterns could be interpreted as evidence for faster divergence on the sex chromosome, i.e. faster-X, or barriers to gene flow on the sex chromosome. However, demographic changes can strongly skew divergence estimates and are not always taken into consideration. We used 224 whole genome sequences representing 36 populations from two Heliconius butterfly clades (H. erato and H. melpomene) to explore patterns of Z chromosome divergence. We show that increased divergence compared to equilibrium expectations can in many cases be explained by demographic change. Among Heliconius erato populations, for instance, population size increase in the ancestral population can explain increased absolute divergence measures on the Z chromosome compared to the autosomes, as a result of increased ancestral Z chromosome genetic diversity. Nonetheless, we do identify increased divergence on the Z chromosome relative to the autosomes in parapatric or sympatric species comparisons that imply post-zygotic reproductive barriers. Using simulations, we show that this is consistent with reduced gene flow on the Z chromosome, perhaps due to greater accumulation of species incompatibilities. Our work demonstrates the importance of constructing an appropriate demographic null model in order to interpret patterns of divergence on the Z chromosome, but nonetheless provides evidence to support the Z chromosome as a strong barrier to gene flow in incipient Heliconius butterfly species.

evolutionary biology↗

bioRxiv2017

Carrion Fly-Derived DNA Metabarcoding Is An Effective Tool For Mammal Surveys: Evidence From A Known Tropical Mammal Community

Metabarcoding of vertebrate DNA derived from carrion flies has been proposed as a promising tool for biodiversity monitoring. To evaluate its efficacy, we conducted metabarcoding surveys of carrion flies on Barro Colorado Island (BCI), Panama, which has a well-known mammal community, and compared our results against diurnal transect counts and camera-trapping. We collected 1084 flies in 29 sampling days, which were pooled into 102 DNA extractions. We then conducted metabarcoding with mammal-specific (16S) and vertebrate-specific (12S) primers targeting mtDNA, and sequenced these amplicons on Illumina MiSeq. For taxonomic assignment, we compared BLAST with the new program PROTAX, and we found PROTAX significantly improved species identifications. We detected 20 mammal, four bird, and one lizard species from carrion fly metabarcoding, all but one of which were previously known from BCI. Fly metabarcoding detected more mammal species than concurrent transect counts (29 sampling days, 13 species) and concurrent camera-trapping (84 sampling days, 17 species), and detected 67% of the number of mammal species documented by 8 years of transect counts and camera-trapping combined, although fly metabarcoding missed several abundant species. This study demonstrates that carrion fly metabarcoding is a powerful tool for mammal biodiversity surveys, and has the potential to detect a broader range of species than more commonly used methods.

zoology↗

bioRxiv2017

Patternize: An R Package For Quantifying Color Pattern Variation

O_LIThe use of image data to quantify, study and compare variation in the colors and patterns of organisms requires the alignment of images to establish homology, followed by color-based segmentation of images. Here we describe an R package for image alignment and segmentation that has applications to quantify color patterns in a wide range of organisms.\nC_LIO_LIpatternize is an R package that quantifies variation in color patterns obtained from image data. patternize first defines homology between pattern positions across specimens either through manually placed homologous landmarks or automated image registration. Pattern identification is performed by categorizing the distribution of colors using an RGB threshold, k-means clustering or watershed transformation.\nC_LIO_LIWe demonstrate that patternize can be used for quantification of the color patterns in a variety of organisms by analyzing image data for butterflies, guppies, spiders and salamanders. Image data can be compared between sets of specimens, visualized as heatmaps and analyzed using principal component analysis (PCA).\nC_LIO_LIpatternize has potential applications for fine scale quantification of color pattern phenotypes in population comparisons, genetic association studies and investigating the basis of color pattern variation across a wide range of organisms.\nC_LI

bioinformatics↗