Host-parasite interactions: Evolutionary genetics of the House Finch- Mycoplasma epizootic

Transcription

1 Host-parasite interactions: Evolutionary genetics of the House Finch- Mycoplasma epizootic Scott V. Edwards Department of Organismic and Evolutionary Biology Harvard University Cambridge, MA USA

2 House Finches and Mycoplasma: a strong host-parasite interaction Mycoplasma gallisepticum escaped chickens and invaded House Finches in the eastern U. S., ~ years later, finches are more resistant and recent bacterial strains are attenuated Natural selection (?) on House Finches by disease Higher survival rates found in: Females versus males Smaller versus larger males Bright males versus dull males Can we identify the genes contributing to survival or susceptibility?

3 Multi-pronged approach to a recently established host-parasite interaction 1. Genetic structure of pre-epizootic house finch populations (AFLPs) 2. Large-scale screen for parasite-induced gene expression in house finches 3. Shifts in allele frequency between pre- and post-epizootic house finches 4. Molecular evolution and host range expansion of the Mycoplasma parasite

4 Recent history of House Finch populations historic range ~1870 bottleneck? 1940 ~200 birds

5 Mycoplasma are obligate parasites and have some of the smallest genomes of any non-virus sequenced

6 Mycoplasma-House Finch History -Mycoplasma gallisepticum escaped chickens and invaded House Finches in the eastern U. S., ~ years later, finches are more resistant to the bacterium and recent strains are attenuated -Have finches evolved resistance? Courtesy Cornell Lab of Ornithology

7 Population and phenotypic consequences of 1994 epidemic Males decline after epidemic Increased redness in males and decreased size after epidemic Sex ratio (M/F) Percent change Pre post epidemic July 1995 August 1996 October Redness Wing chord (mm) Bill (mm) Tarsus (mm) Males Tail Lengt h(mm) Weight (g) Wing chord (mm) Bill (mm) Tarsus (mm) Females Weigh t (g) From Nolan, P. M., G.E. Hill and A. M. Stoehr Proc. R. Soc. Lond. B.265:

8 AFLPs: House Finch are moderately structured with little evidence for genetic bottlenecks 163 individuals, 16 populations, 3 primer combinations, 166 polymorphic bands, 61% polymorphic bands Distribution of variation (AMOVA) Among individuals w/in pops. 70.7% 8.1% 21.2% Nucleotide diversity (estimated number of substitutions per 1000 sites) Nucleotide diversity original range introduced range CA CA TX AR CO WAMex. HI MI ME NY OH MD PA AL Can. Among pops. w/in subspecies (native range) Among subspecies (native range) Wang, Z., Hill, G. E., Baker, A. J. & Edwards, S. V. (2003) Evolution 57,

9 Tripartite structure of House Finch populations suggested by assignment test of AFLP data (program STRUCTURE: J. Pritchard et al Genetics 155: ) Western U.S. Hawaii Eastern U. S. Wang, Z., Hill, G. E., Baker, A. J. & Edwards, S. V. (2003) Evolution 57,

10 Suppression subtractive hybridization Experimental cdna, split into two pools A PCR method for differentially amplifying transcripts that differ in expression in two cell populations Often used in plant studies; a useful alternative to microarrays cdnas differentially expressed cdnas cdnas shared between control and tester normalization driver tester 1 tester 2 (control) ligate primers ( ) to two cdna pools hybridization 1 hybridization 2 fill in ends selectively amplify

11 Example macroarray results Probe identical filters with RNA from infected and uninfected birds Distinct hybridizations - differentially expressed genes Common hybridizations -- noise C A identical filters (A + B, C + D) B Reciprocally subtracted probes (A vs. B, C vs. D) D

12 Sequencing suggests change in expression for heat shock and immune system genes Additional upregulated genes Number of sequenced clones Granzyme A Additional downregulated genes Mhc class II Wang et al. (2006) Mol. Ecol. 15,

13 Preliminary network of genes induced by infection infection Mycoplasma Healthy Finch infected finch HSP90 TIM1 Granzyme A Mhc class II, invariant chain chaperone with diverse substrates apoptosis elongation factor 1α mitochondrial degredation COI, COIII, NADH4 Host modulation or parasite subversion of immune response? Wang et al. (2006) Mol. Ecol. 15,

14 Museum specimens permit temporal comparison of genetic diversity pre- and post-epidemic House Finch populations Recently exposed California? Michigan Exposed 1990s present control comparison Unexposed Alabama diachronic comparison late 1980s Royal Ontario Museum (A. J. Baker) Unexposed

15 Mhc class I crystal structure α1 domain [ peptide binding region (PBR) peptide α2 domain β2 microglobulin

16 Both increases and decreases in diversity are predicted by evolution of resistance in house finches Finch with conjunctivitis Mhc class II molecules Healthy Finch homozygote heterozygote Foreign pathogen

17 Little evidence for change in heterozygosity (θ) at an Mhc class II locus between pre- and post-epidemic samples θ Michigan late 1980s Michigan 2000 Alabama 1994 Alabama 2000 California late 1980s California 1999 Hess, C. M., Wang, Z. & Edwards, S. V. (2007) Genetica 129,

18 However, rapid shifts in frequency observed at some peptide-binding codons * MHC class II peptide binding codon Hess, C. M., Wang, Z. & Edwards, S. V. (2007) Genetica 129,

19 The Mycoplasma gallisepticum genome: ~0.99 Mb Papazisi, L., et al. (2003) Microbiology 149,

20 Variation in genome size among House Finch (HF) and Turkey (TK) isolates of Mycoplasma SmaI EagI HF GA 1995 TK GA 1973 HF GA 1995 TK GA kb kb kb kb * 48.5 kb 23.1 kb Courtesy Wendy Smith, unpubl. data

21 Recent host shift of Mycoplasma gallisepticum to house finches (HF) - but how recent? 99% 100% 89% 95% 100% HF-TK clade 64% 0.05 substitutions/site TK GA 1973 CK SC 2000 CK GA 1974 CK GA 1964 CK Vaccine TK NC 1995 HF GA 1995 HF GA 1995 HF AL 2001 TK IN 2000 HF VA 1994 TK IN 2000 TK VA 1996 M. penetrans M. genitalium M. synoviae M. hypopneumoniae Bacillus subtilis TK = turkey CK = chicken HF = House Finch Mycoplasma gallisepticum Maximum likelihood tree, ~5200 bp RpoB and fusa genes Courtesy Wendy Smith, unpubl. data

22 Empirical conclusions Pre-epizootic House Finch structure AFLPs suggest significant but mild population differentiation Parasite - induced gene expression House Finches show up- and down-regulation of key immune system genes upon experimental infection Diachronic allele frequency shifts in house finch populations Little evidence for reductions in diversity but some evidence for allele frequency shifts at key immune system genes Parasite evolution DNA sequence information provides a detailed view of Mycoplasma history

23 Conservation implications A double invasion Range expansions in both hosts and parasites results in novel evolutionary pressures Microbial host range expansion Adaptation of Mycoplasma gallisepticum to a novel host could result in yet further increases in host range in wild birds Implications for infectious disease biology Pathogens can spread across the country in a matter of years A number of unresolved issues in the role of genetic diversity in regulating parasite expansion

24 Acknowledgments MHC evolution Christopher Hess, U. Washington AFLPs, macroarray analysis Zhenshan Wang, U. Washington Kristy Farmer, Geoff Hill, Auburn U. Funding NSF Mycoplasma evolution Wendy Smith & Colin Dale, U. Utah and Auburn U.