Cheryl L. Tarr

Evolution on a volcanic conveyor belt: using phylogeographic reconstructions and K-Ar-based ages of the Hawaiian Islands to estimate molecular evolutionary rates

The Hawaiian Islands form as the Pacific Plate moves over a ‘hot spot’ in the earth’s mantle where magma extrudes through the crust to build huge shield volcanos. The islands subside and erode as the plate carries them to the north‐west, eventually to become coral atolls and seamounts. Thus islands are ordered linearly by age, with the oldest islands in the north‐west (e.g. Kauai at 5.1 Ma) and the youngest in the south‐east (e.g. Hawaii at 0.43 Ma). K–Ar estimates of the date of an island’s formation provide a maximum age for the taxa inhabiting the island. These ages can be used to calibrate rates of molecular change under the following assumptions: (i) K–Ar dates are accurate; (ii) tree topologies show that derivation of taxa parallels the timing of island formation; (iii) populations do not colonize long after island emergence; (iv) the coalescent point for sister taxa does not greatly predate the formation of the colonized younger island; (v) saturation effects and (vi) among‐lineage rate variation are minimal or correctable; and (vii) unbiased standard errors of distances and regressions can be estimated from multiple pairwise comparisons. We use the approach to obtain overall corrected rate calibrations for: (i) part of the mitochondrial cytochrome b gene in Hawaiian drepanidines (0.016 sequence divergence/Myr); (ii) the Yp1 gene in Hawaiian Drosophila (0.019/Myr Kambysellis et al. 1995); and (iii) parts of the mitochondrial 12S and 16S rRNA and tRNAval in Laupala crickets (0.024–0.102/Myr, Shaw 1996). We discuss the reliability of the estimates given the assumptions (i–vii) above and contrast the results with previous calibrations of Adh in Hawaiian Drosophila and chloroplast DNA in lobeliods.

Cryptic genetic variation and paraphyly in ravens

Widespread species that are morphologically uniform may be likely to harbour cryptic genetic variation. Common ravens (Corvus corax) have an extensive range covering nearly the entire Northern Hemisphere, but show little discrete phenotypic variation. We obtained tissue samples from throughout much of this range and collected mitochondrial sequence and nuclear microsatellite data. Our study revealed a deep genetic break between ravens from the western United States and ravens from throughout the rest of the world. These two groups, the ‘California clade’ and the ‘Holarctic clade’ are well supported and over 4% divergent in mitochondrial coding sequence. Microsatellites also reveal significant differentiation between these two groups. Ravens from Minnesota, Maine and Alaska are more similar to ravens from Asia and Europe than they are to ravens from California. The two clades come in contact over a huge area of the western United States, with mixtures of the two mitochondrial groups present in Washington, Idaho and California. In addition, the restricted range Chihuahuan raven (Corvus cryptoleucus) of the south–west United States and Mexico is genetically nested within the paraphyletic common raven. Our findings suggest that the common raven may have formerly consisted of two allopatric groups that may be in the process of remerging.

MITOCHONDRIAL-DNA VARIATION AND EVOLUTIONARY RELATIONSHIPS IN THE AMAKIHI COMPLEX

AssRAcT.-An analysis of restriction-site variation in mitochondrial DNA was conducted to examine relationships among five taxa in one group of honeycreepers-the amakihi complex (genus Hemignathus). We analyzed 35 ingroup and 3 outgroup samples. Tree topologies, based on both distance and parsimony methods, grouped taxa into two distinct lineages: the virens-wilsoni lineage; and the chloris-stejnegeri-parvus group. Inter-island sequence divergence (average d,X = 0.0368) is considerably higher than intra-island variation (mean d, = 0.0035), and is higher than average for avian species. Variability (measured as both nucleotide diversity and maximum divergence between haplotypes) differs among island populations. Molecular evolutionary rates were calibrated on the basis of maximum island age estimates; sequence divergence in this lineage is approximately 2% per million years. The relationships within the chloris-stejnegeri-parvus clade generally are consistent with the previously proposed model of double invasion. Genetic distances and the pattern of relationships among amakihi taxa indicate that species status for H. v. chloris and H. v. stejnegeri may be warranted. Received 15

Founder events and variation at microsatellite loci in an insular passerine bird, the Laysan finch (Telespiza cantans)

Historically documented founder events provide opportunities to assess the effects of population size reductions on genetic variation, but the actual magnitude of genetic change can be measured only when direct comparisons can be made to the source or ancestral population. We assayed variation at nine microsatellite loci in the translocated population of the Laysan finch (Telespiza cantans) at Pearl and Hermes reef (PHR), and compared the level of variation to that in the source population on Laysan Island. Heterogeneity in allele frequencies was highly significant at eight of the nine loci, primarily as a result of fluctuations in allele frequencies in the three PHR populations. Intra‐ and interpopulational measures of genetic diversity generally matched predictions based on the well‐documented history of three islet populations at PHR: significantly lower numbers of alleles and polymorphic loci, as well as higher pairwise FST values and genetic distance, were observed for the two populations that underwent severe size reductions. Changes in heterozygosity at single loci were unpredictable, as both significant increases and decreases were observed in founder populations. A significant excess of heterozygotes was found in two populations and was highly significant over all four finch populations (P < 0.003). Estimates of effective population size from temporal changes in heterozygosity and allele frequencies were very small (Ne≤ 30) as a result of the founding events and the constraints of islet area on population numbers. We concluded that the PHR population is not adequate as a secondary genetic reserve for T. cantans, and an alternative refuge needs to be established.

Natural selection of the major histocompatibility complex (Mhc) in Hawaiian honeycreepers (Drepanidinae)

The native Hawaiian honeycreepers represent a classic example of adaptive radiation and speciation, but currently face one the highest extinction rates in the world. Although multiple factors have likely influenced the fate of Hawaiian birds, the relatively recent introduction of avian malaria is thought to be a major factor limiting honeycreeper distribution and abundance. We have initiated genetic analyses of class II β chain Mhc genes in four species of honeycreepers using methods that eliminate the possibility of sequencing mosaic variants formed by cloning heteroduplexed polymerase chain reaction products. Phylogenetic analyses group the honeycreeper Mhc sequences into two distinct clusters. Variation within one cluster is high, with dN > dS and levels of diversity similar to other studies of Mhc (B system) genes in birds. The second cluster is nearly invariant and includes sequences from honeycreepers (Fringillidae), a sparrow (Emberizidae) and a blackbird (Emberizidae). This highly conserved cluster appears reminiscent of the independently segregating Rfp‐Y system of genes defined in chickens. The notion that balancing selection operates at the Mhc in the honeycreepers is supported by transpecies polymorphism and strikingly high dN/dS ratios at codons putatively involved in peptide interaction. Mitochondrial DNA control region sequences were invariant in the i’iwi, but were highly variable in the ‘amakihi. By contrast, levels of variability of class II β chain Mhc sequence codons that are hypothesized to be directly involved in peptide interactions appear comparable between i’iwi and ‘amakihi. In the i’iwi, natural selection may have maintained variation within the Mhc, even in the face of what appears to a genetic bottleneck.

Mating System of the Dusky Antbird, a Tropical Passerine, as Assessed by DNA Fingerprinting

We studied the genetic mating system of the Dusky Antbird (Cercomacra tyrannina) in Panama using multilocus DNA fingerprinting. We found no evidence of extra-pair fertilization (EPF) in 15 offspring of nine families. We also found no evidence of intraspecific brood parasitism (ISBP) for 13 offspring in eight families.

Population boundaries and genetic diversity in the endangered Mariana crow (Corvus kubaryi)

The Mariana crow (Corvus kubaryi) is an endangered species that is restricted to the islands of Guam and Rota in the Mariana archipelago. Predation by the introduced brown tree snake (Boiga irregularis) has decimated bird populations on Guam, and the crow population there is the last wild remnant of the endemic forest avifauna. The population on Guam is critically endangered and, despite intensive management, the population has continued to decline. Additional management options include intermixing the Guam and Rota populations, but such options are best evaluated within a population genetics framework. We used three types of molecular markers to assay genetic variation in the Mariana crow: mitochondrial DNA (mtDNA) sequences, minisatellites and microsatellites. The two populations could be differentiated by mtDNA sequencing and they differed in allele frequencies at nuclear markers. Thus, the populations could be designated as evolutionarily significant units. However, the Guam population is genetically more diverse than the Rota population, and its survival probability if managed separately is very low. All markers did indicate that the two populations are closely related and separated by a shallow genealogical division. Intermixing the populations is justified by two rationales. First, the apparent population differences may result from recent human activities. Second, a greater amount of genetic information may be preserved by joint management. The translocation of birds from Rota to Guam has begun, but strategies that will ensure maintenance of the variation in the Guam population warrant further exploration.

Microsatellite variation in simulated and natural founder populations of the Laysan finch (Telespiza cantans)

Historical and demographic data were used in a computer model tosimulate neutral genetic change in populations of the Laysanfinch (Telespiza cantans), an insular passerine bird that hasundergone documented founder events at Pearl and Hermes reef(PHR). Measures of genetic variation in the natural PHRpopulations generally matched those in the simulated populations,except that heterozygosity on Southeast Island was lower than themodel predicted, and the heterozygote excess in the naturalpopulations had a low probability of occurrence in the simulatedpopulations. The estimate of effective population size (Ne) fromthe stochastic demographic model matched the estimate fromgenetic data for two populations, but the demographic estimatewas higher than the genetic estimate for Southeast Island. Smallfounder number was rejected as a possible explanation for thereduced genetic variation on Southeast. We suggest that Ne wasoverestimated in part because we assumed seasonal variance inreproductive success. Additional variance components need to bemeasured in the field and incorporated into the model. Accounting for the heterozygote excess also requires furthertheoretical and field investigations. Possible explanations forthe excess include inbreeding depression, incest avoidance, andthe effect of polygyny on heterozygote excess in smallpopulations. We concluded that the Pearl and Hermes reefpopulation will continue to lose genetic variation at a highrate, and translocations from the native population on Laysan maybe required to maintain a viable population on the reef.