четверг, 7 мая 2020 г.

How gene flow between species influences the evolution of Darwin's finches

Despite the traditional view that species do not exchange genes by hybridisation, recent studies show that gene flow between closely related species is more common than previously thought. A team of scientists from Uppsala University and Princeton University now reports how gene flow between two species of Darwin's finches has affected their beak morphology. The study is published in Nature Ecology and Evolution.

How gene flow between species influences the evolution of Darwin's finches
The common cactus finch has a pointed beak adapted to feed on cactus whereas the medium ground finch has
 a blunt beak adapted to crush seeds. Their hybrid progeny have an intermediate beak morphology adaptive
under certain environmental conditions as explained in this paper [Credit: Sangeet Lamichhaney,
Rosemary and Peter Grant]

Darwin's finches on the Galapagos Islands are an example of a rapid adaptive radiation in which 18 species have evolved from a common ancestral species within a period of 1-2 million years. Some of these species have only been separated for a few hundred thousand years or less.

Rosemary and Peter Grant of Princeton University, co-authors of the new study, studied populations of Darwin's finches on the small island of Daphne Major for 40 consecutive years and observed occasional hybridisation between two distinct species, the common cactus finch and the medium ground finch. The cactus finch is slightly larger than the medium ground finch, has a more pointed beak and is specialised to feed on cactus. The medium ground finch has a blunter beak and is specialised to feed on seeds.

"Over the years, we observed occasional hybridisation between these two species and noticed a convergence in beak shape. In particular, the beak of the common cactus finch became blunter and more similar to the beak of the medium ground finch," say Rosemary and Peter Grant. "We wondered whether this evolutionary change could be explained by gene flow between the two species."

How gene flow between species influences the evolution of Darwin's finches
Medium ground finch with its blunt beak. This particular bird has been banded by Rosemary and Peter Grant during their
field studies on Daphne Major. Reproduced with permission from K. Thalia Grant, and Princeton University Press,
which first published the remaining images in 40 Years of Evolution (P. R. Grant & B. R. Grant, 2014)
[Credit: Peter R. Grant and B. Rosemary Grant]

"We have now addressed this question by sequencing groups of the two species from different time periods and with different beak morphology. We provide evidence of a substantial gene flow, in particular from the medium ground finch to the common cactus finch," explains Sangeet Lamichhaney, one of the shared first authors and currently Associate Professor at Kent State University.

"A surprising finding was that the observed gene flow was substantial on most autosomal chromosomes but negligible on the Z chromosome, one of the sex chromosomes," says Fan Han, Uppsala University, who analysed these data as part of her PhD thesis. "In birds, the sex chromosomes are ZZ in males and ZW in females, in contrast to mammals where males are XY and females are XX."

"This interesting result is in fact in excellent agreement with our field observation from the Galapagos," explain the Grants. "We noticed that most of the hybrids had a common cactus finch father and a medium ground finch mother. Furthermore, the hybrid females successfully bred with common cactus finch males and thereby transferred genes from the medium ground finch to the common cactus finch population. In contrast, male hybrids were smaller than common cactus finch males and could not compete successfully for high-quality territories and mates."

How gene flow between species influences the evolution of Darwin's finches
Common cactus finch with its pointed beak feeding on the Opuntia cactus
[Credit: Lukas Keller]

This mating pattern is explained by the fact that Darwin's finches are imprinted on the song of their fathers so that sons sing a song similar to their father's song and daughters prefer to mate with males that sing like their fathers. Furthermore, hybrid females receive their Z chromosome from their cactus finch father and their W chromosome from their ground finch mother. This explain why genes on the Z chromosome cannot flow from the medium ground finch to the cactus finch via these hybrid females, whereas genes in other parts of the genome can, because parents of the hybrid contribute equally.

"Our data show that the fitness of the hybrids between the two species is highly dependent on environmental conditions which affect food abundance," says Leif Andersson of Uppsala University and Texas A&M University. "That is, to what extent hybrids, with their combination of gene variants from both species, can successfully compete for food and territory. Therefore, the long-term outcome of the ongoing hybridisation between the two species will depend on environmental factors as well as competition."

"One scenario is that the two species will merge into a single species combining gene variants from the two species, but perhaps a more likely scenario is that they will continue to behave as two species and either continue to exchange genes occasionally or develop reproductive isolation if the hybrids at some point show reduced fitness compared with purebred progeny. The study contributes to our understanding of how biodiversity evolves," Andersson concludes.

* This article was originally published here

New findings on the Byzantine necropolis Tell es-Sin in Syria

A study published in the journal Bioarcheology of the Near East reveals the characteristics of the population that was buried in the Tell es-Sin necropolis, a Byzantine site dated between the 5th and 7th centuries that is located in Syria, on the left bank from the Euphrates River.

New findings on the Byzantine necropolis Tell es-Sin in Syria
Tell es-Sin represents one of the most important necropolis from the Fertile Crescent
 to the Near East [Credit: University of Barcelona]

The main authors of the new anthropological work on this site are researchers Laura Martinez, from the Faculty of Biology at the University of Barcelona (UB), and Ferran Estebaranz Sanchez, from the Faculty of Biosciences at the Autonomous University of Barcelona (UAB).

Tooth Mount in Ancient Syria

The Tell es-Sin site – from the Arab, ‘Monte del Diente’ – occupies an area of ??twenty-five hectares located in the middle of a passage area for the ancient Byzantine and Persian Sassanid armies. It is divided into the acropolis, the lower city and the necropolis, which occupies seven hectares. It is located near the southeast of the current city of Deir ez-Zor – on the border between Syria and Iraq – and is considered a kastron, that is, an outpost with both administrative and military functions. The size of the site, its urban structure and its fortified nature suggest that it would be an old polis whose name is still unknown.

New findings on the Byzantine necropolis Tell es-Sin in Syria
Tell es-Sin is an archaeological site located in the middle of a transit area for the ancient Byzantine forces
and the Persian Sassanids [Credit: University of Barcelona]

Tell es-Sin is one of the most important necropolis of the Fertile Crescent in the Middle East, “but very little is still known about it,” the authors note. The new work wants to deepen the knowledge of the populations of the border of the Byzantine Empire during the 6th and 7th centuries, a period of which the necropolis and skeletal remains are scarce.

A fortification in the middle of the military map of the Middle East

Mesopotamia was a strategic defensive region against both Persian and Arab incursions and invasions. In this context, Tell es-Sin could have been affected by the territorial and military reorganization carried out by Emperor Justinian, who promoted the fortification of the limes populations in the middle of the 6th century of our era, “explains Laura Martinez, professor at the Department of Evolutionary Biology, Ecology and Environmental Sciences of the Faculty of Biology and first author of the study.

The first archaeological excavations of the Byzantine necropolis of Tell as-Sin date from 1978 and were led by Asad Mahmoud, general director of Antiquities and Museums at Deir ez-Zor at the time. In 2005, the investigative work of the first Syrian-Spanish archaeological mission – coordinated by the University of La Coruna – in the area highlighted the importance of the necropolis of the Tell es-Sin site, which was part of the Diocletianus limes in the East along with those of Tell es-Kasr and Circesium (current city of Buseira). In total, experts have identified 170 hypogea in a necropolis that could contain up to 1,000 graves.

Byzantine tombs and archaeology in Syrian territory

As Ferran Estebaranz Sanchez explains, “the samples from Tell es-Sin constitute a heterogeneous and biased set of skeletal remains that correspond to tombs looted during the course of time; Using traditional biometric methods, this anthropological study wanted to provide information on the sex, age of death, height and other morphological variables of the individuals found in the site.

New findings on the Byzantine necropolis Tell es-Sin in Syria
Samples from Tell es-Sin represent an heterogeneous and biased series of skeleton remains corresponding
 to tombs that were sacked during the years [Credit: University of Barcelona]

The sample analyzed – only a small part of the total number of Tell es-Sin burials – includes human remains from ten hypogea excavated by the Syrian-Spanish mission. In total, a total of 71 individuals have been analyzed (at least eighteen would correspond to men and twelve to women).

According to experts, no bias regarding sex or age has been observed in the remains studied, and highlights the lack of children compared to other places (they could have been buried in other niches at the entrance to the tomb). Also, there are at least one to five individuals buried within each niche (the average is three bodies per niche, including subadults and adults), according to the typical collective burial model of ancient Syria.

Despite the fragmented state of the remains, the team was able to estimate the height of most individuals. “The estimated mean height from the long bones of the upper limb was 174.5 cm for men and 159.1 cm for women. These values are very similar to those estimated from the diameter of the head of the femur: 176.1 cm for male individuals and 164.5 cm for female individuals, “comments Estebaranz Sanchez.

“In conclusion,” he continues, “the estimated height for the Byzantine population of Tell es-Sin is similar to that of other contemporary Byzantine populations.”

New findings on the Byzantine necropolis Tell es-Sin in Syria
According to the experts, they did not observe bias regarding sex or age in the studied remains
[Credit: University of Barcelona]

About 25% of the individuals presented with orbitalia screening, and 8.5%, porotic hyperostosis, alterations of the cranial bones traditionally associated with anemia or iron deficiency, rickets, infection or other inflammatory conditions.

The prevalence of degenerative joint diseases was also low, the study points out. Regarding the dental sample, only 2.8% of the teeth had cavities, a value clearly lower than that of other contemporary Byzantine sites in the region that could be related to the low sample analyzed at the Tell es-Sin site.

Tell es-Sin: the end of a settlement with the arrival of Islam

The end of the Tell es-Sin seat – in the first quarter of the 7th century AD – coincided with the wars against the Sassanid Persians and the Arab tribes of Islam. Despite the conditions of the Tell es-Sin deposit and the current situation in the region – following the occupation by ISIS – the discovery and excavation of unsacked graves in the future is crucial to deepen the knowledge of this population.

“For this reason, we are currently analyzing the pattern of oral microstriation to be able to infer the diet of the population and thus complete the biocultural model of the border populations with the great empires of antiquity,” conclude Laura Martinez and Ferran Estebaranz Sanchez.

* This article was originally published here


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