Tuberculosis and HIV coinfection
SIV-infected cells in lung granuloma from macaque with extensive TB.
One in three people harbor latent Mycobacterium tuberculosis (Mtb), the bacterium that causes tuberculosis (TB), but latent Mtb can trigger active TB. Coinfection with HIV can speed the progression of latent TB to active TB, a process likely driven by a depletion of CD4+ T cells. To study the immunological regulation of latent TB infection by HIV, Taylor Foreman et al. (pp. E5636–E5644) infected macaques with Mtb and simian immunodeficiency virus (SIV), mimicking Mtb and HIV coinfection in humans. In a majority of animals, Mtb replication rapidly reactivated and progressed to active TB, and pathologies associated with SIV increased. Despite the decrease in pulmonary CD4+ T cells in all coinfected macaques, one-third of the animals maintained TB latency. For this cohort, an increase in protective immune responses, mediated by CD8+ memory T-cell proliferation and increased B-cell responses, was associated with limited Mtb replication. According to the authors, the findings may provide insights into natural immunity to Mtb and could help guide the development of vaccines and immunotherapies for TB and HIV. — L.C.
Herbivorous mammals and origins of African savanna
Kudu are large mammal browsers, a group linked to the evolution of spinescence.
A group of plants called C4 grasses dominate savanna land and emerged around 30 million years ago during the Oligocene Epoch. However, savannas did not become a major Earth biome until nearly 20 million years later during the Miocene Epoch. One explanation for the expansion of savanna land on the African continent relates to an increase in C4 grasses that promote fire. Tristan Charles-Dominique et al. (pp. E5572–E5579) found that herbivore-adapted savannas in Africa likely evolved before fire-maintained savannas. The authors analyzed spinescence, or the presence of spines on woody plants, as a marker of mammalian herbivory, and examined the present distribution and dated phylogeny of more than 1,800 African tree species. The authors found that spinescence evolved in unrelated plant lineages at least 55 times over the last 16 million years and was mainly associated with large browser mammals and medium-sized mixed feeder mammals. Moreover, the diversification of spiny plants coincided with the diversification of bovid animals, suggesting that bovid diversification altered woody plant community composition and structure. According to the authors, herbivory pressure exerted by an influx of bovids may have provided the impetus for the initial spread of savannas across Africa during the mid-Miocene, long before fire-related savanna expansion that occurred during the late Miocene. — C.S.
Glaciation and kiwi diversification
Brown kiwi in Willowbank Wildlife Reserve, Christchurch, New Zealand. Image courtesy of Wikimedia Commons/Allie_Caulfield.
The role of Pleistocene glaciation in promoting species diversification is unclear, with molecular dating suggesting that most species originated before the most recent glacial cycles. However, other evidence suggests that glaciers promoted diversification in adjacent habitats. Jason Weir et al. (pp. E5580–E5587) combined DNA sequences from living kiwi, flightless birds endemic to New Zealand, with previously published sequences from fossilized kiwi to determine the timing and rates of kiwi diversification. The authors identified 17 genetically distinct lineages within the five currently recognized kiwi species, 11 of which are extant. Using a population genetic modeling approach, the authors estimated the timing of splits between the various lineages. Most of these splitting events coincided with the major glacial maxima of the Middle and Late Pleistocene Epoch, when glaciers fragmented much of New Zealand into small ice-free zones, which lie adjacent to the present-day geographic ranges of kiwi lineages. During the Middle and Late Pleistocene glacial periods, the rate at which new kiwi lineages originated increased more than five-fold. Estimates of effective population size over time indicate a large drop during the last glacial period for most lineages. According to the authors, the results suggest that Pleistocene glacial cycles promoted kiwi diversification, and support a role for glacial cycles in driving speciation near glaciated regions. — B.D.