Existing and emerging pathogens pose unusual challenges for conservation because of their potential to drive rapid changes in the numerical abundance and genetic composition of wild host populations. An increasing number of studies indicate that host genetic diversity plays an important role in buffering populations against widespread epidemics, and that parasites represent powerful selective agents in natural populations. The observation that infectious diseases might be both mitigated by and rapidly change the genetic composition of host populations gives new significance to the role of host genetic diversity in species conservation. Less clear is the role that pathogen evolutionary change plays in the emergence and spread of new diseases, but recent examples indicate that humans might be selecting unknowingly for rapid changes in pathogen biology through habitat fragmentation, climate shifts and environmental pollution. Although the risks they pose to endangered species are apparent, pathogens and other natural enemies can be a driving force behind species and genetic diversity in natural populations, and preserving interacting networks of coevolving populations should enable hosts to respond better to future disease threats.

Rapid evolutionary dynamics and disease threats to biodiversity

The switching of parasitic organisms to novel hosts, in which they may cause the emergence of new diseases, is of great concern to human health and the management of wild and domesticated populations of animals. We used a phylogenetic approach to develop a better statistical assessment of host switching in a large sample of vector-borne malaria parasites of birds (Plasmodium and Haemoproteus) over their history of parasite-host relations. Even with sparse sampling, the number of parasite lineages was almost equal to the number of avian hosts. We found that strongly supported sister lineages of parasites, averaging 1.2% sequence divergence, exhibited highly significant host and geographical fidelity. Event-based matching of host and parasite phylogenetic trees revealed significant cospeciation. However, the accumulated effects of host switching and long distance dispersal cause these signals to disappear before 4% sequence divergence is achieved. Mitochondrial DNA nucleotide substitution appears to occur about three times faster in hosts than in parasites, contrary to findings on other parasite-host systems. Using this mutual calibration, the phylogenies of the parasites and their hosts appear to be similar in age, suggesting that avian malaria parasites diversified along with their modern avian hosts. Although host switching has been a prominent feature over the evolutionary history of avian malaria parasites, it is infrequent and unpredictable on time scales germane to public health and wildlife management.

/pdf/diversification-and-host-switching-in-avian-malaria-3u4j6jguf5.pdf

Diversification and host switching in avian malaria parasites

Taxon cycles are sequential phases of expansion and contraction of the ranges of species, associated generally with shifts in ecological distribution. The important contribution of the taxon cycle to biogeographical analysis is its emphasis on evolutionary and ecological interactions among colonizing and resident species, which influence their extinction dynamics and establish patterns of geographical distribution. Taxon cycles were inferred originally from the distribution of species across island archipelagos, where a correlation was noted between gaps in island occupancy and the degree of phenotypic differentiation. This pattern implied that phases of colonization were followed by range contraction, while endemic Antillean species that were undifferentiated between islands suggested secondary expansion and the beginning of a new cycle. This interpretation was met with scepticism, but reconstruction of phylogenetic relationships from gene sequences has now permitted us to characterize taxon cycles in Lesser Antillean birds. The relative timing of phases of the cycle can be deduced from genetic divergence between island populations. We have found that taxon cycles have periods in the order of 10 6 years and that cycles in different lineages occur independently of each other and independently of Pleistocene climate cycles. Individual island populations may persist for several millions of years on the larger islands of the Lesser Antilles; occasional expansion phases lead to the replacement of island populations that have disappeared, thus reducing the archipelago-wide rate of extinction to nil. What drives taxon cycles is unknown, but we speculate that they may be caused by co-evolution with enemy populations, and a probable mechanism would involve infrequent mutations influencing parasite virulence and avian host disease resistance. Taxon cycles undoubtedly occur on continents, but the geographical configuration of island archipelagos reveals more clearly their presence and invites their study.

/pdf/the-concept-of-the-taxon-cycle-in-biogeography-2w2dcm5pkz.pdf

The concept of the taxon cycle in biogeography

One of the best-known general patterns in island biogeography is the species–isolation relationship (SIR), a decrease in the number of native species with increasing island isolation that is linked to lower rates of natural dispersal and colonization on remote oceanic islands However, during recent centuries, the anthropogenic introduction of alien species has increasingly gained importance and altered the composition and richness of island species pools We analyzed a large dataset for alien and native plants, ants, reptiles, mammals, and birds on 257 (sub) tropical islands, and showed that, except for birds, the number of naturalized alien species increases with isolation for all taxa, a pattern that is opposite to the negative SIR of native species We argue that the reversal of the SIR for alien species is driven by an increase in island invasibility due to reduced diversity and increased ecological naivete of native biota on the more remote islands

https://www.pnas.org/content/pnas/115/37/9270.full.pdf

Remoteness promotes biological invasions on islands worldwide.

We examined temporal and geographical distribution of Haemoproteus sp. and Plasmodium sp. parasites in Swedish willow warblers, Phylloscopus trochilus. Parasite lineages were detected with molecular methods in 556 birds from 41 sites distributed at distances up to 1,500 km. Two mitochondrial lineages of Haemoproteus sp. were detected, WW1 in 56 birds and WW2 in 75 birds, that differed by 5.2% sequence divergence. We discuss the reasons behind the observed pattern of variation and identify 3 possible causes: (1) variation in the geographic distribution of the vector species, (2) the degree of parasite sharing with other bird species coexisting with the willow warbler, and (3) timing of transmission. Our results support a fundamental and rarely tested assumption of the now classical Hamilton–Zuk hypothesis of sexual selection, namely, that these parasites vary in both time and space. Such fluctuations of parasites and the selection pressure they supposedly impose on the host population are likely to maintai...

Temporal and spatial variation of hematozoans in Scandinavian willow warblers.

Patterns of parasitism in insular avian communities can provide insight into processes that maintain host-parasite associations. On one hand, replicable relationships within evolutionarily independent communities of the same host and parasite taxa would indicate that these interactions are stable over time. On the other hand, unique ecological conditions on each island, sporadic colonizations and extinctions, plus new genetic variation would lead to island-specific host-parasite relationships. We examined the distribution of three parasitic taxa among avian host species on three islands in the Lesser Antilles: St. Lucia, Martinique, and Dominica. Blood protozoa of the genus Haemoproteus were found in 34% of host individuals examined. A significant species effect, but no significant island effect, was observed, suggesting an ecologically stable and replicable host-parasite asso- ciation. Cysts of the tissue-dwelling protozoan genus Sarcocystis were observed in 4% (9% on Dominica) of host individuals and were significantly associated with ground foraging. Epithelial lesions characteristic of avian papilloma virus were recorded in 4% of host individuals on Martinique only. The pattern of infection with papilloma virus or Sarcocystis (significant island effect) indicated that host species on a particular island are linked in transmission webs of these parasites. Such island-specific associations suggest a role for either history or unique local ecology in host-parasite associations. There was a statistically significant interaction between island and species effects in the prevalence of Haemoproteus. This may stem from the independent evolution of host- parasite interactions in the different island populations. We were able to assess the extent of genetic divergence of the host species by analysis of mitochondrial ATPase 6,8 sequences. There was little genetic divergence between island populations of the host species. There- fore, the variation in Haemoproteus prevalence is not likely to be related to genetic dif- ferentiation of the host populations. Birds infected with Haemoproteus exhibited elevated leukocyte levels indicative of immunological control of the parasite. After statistically controlling for the intensity of Haemoproteus infection and host species, leukocyte levels varied significantly among is- lands on which the host resided. This is consistent with the idea that insular avian com- munities are linked by transmission webs of parasites having broad host specificity.

/pdf/island-and-taxon-effects-in-parasitism-and-resistance-of-517dty5k7p.pdf

Island and taxon effects in parasitism and resistance of lesser antillean birds

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Island and taxon effects in parasitism and resistance of lesser antillean birds