Study suggests that having common ancestors can jeopardize fertility for generations

When it comes to the architecture of the human genome, it's only a matter of time before harmful genes -- genes that could compromise future generations -- arise in a population. These mutations accumulate in the gene pool, primarily affected by a population's size and practices like marrying within a small community, according to researchers.

But much of the information about the effects of a population's mutation load is based on genetic theory, with limited direct evidence concerning the effects on evolutionary fitness, or fertility.

New research from University of California, Davis, provides rare direct evidence showing that increased homozygosity -- meaning two identical alleles in a genome -- leads to negative effects on fertility in a human population. The paper was published Oct. 17 in the Proceedings of the National Academy of Sciences journal.

"People have known since Darwin that if you take people who are first cousins and they have children together, the children are more likely to develop certain diseases or be less healthy," said Brenna Henn, an associate professor of anthropology in the College of Letters and Science at UC Davis.

The research assesses the consequences of homozygosity among Namibia's Himba community, an isolated, agro-pastoralist population in which marriage between people with the same ancestor occurs. The research was led by Natalie Swinford, who received her doctoral degree in 2022 in evolutionary anthropology and human population genetics, and Henn.

"They're what we call an 'endogamous population,' meaning people are meeting their partners just from within that Himba group," said Henn. "They also have a unique system of marriage and reproduction, where men and women can have multiple boyfriends or girlfriends during their marriage. That means there are a lot of half-siblings in the population. That's a unique feature and it means that we can leverage that social structure to look at different genetic effects."

Echoes in the genome

In the study, the team gathered genetic data from 681 individuals from the Himba population. Genetic analyses revealed that the Himba have genetic markers that show higher levels of inbreeding.

Known as runs of homozygosity, or ROH, these markers are multiple and particularly long in the genomes of the analyzed Himba, which indicates that their parents had a high likelihood of sharing an ancestor.

While the Himba population has historically exhibited a preference for consanguinity, Henn and Swinford were surprised to find that none of the individuals in their sample population had parents who were actually first cousins. The lengths of the ROH in the genomes indicated otherwise.

The researchers found that these genetic effects can pool and accumulate over time. So, bottleneck events, like a decrease in population that leads to inbreeding, can have genetic echoes that don't manifest until generations later. The researchers concluded that such events occurred within the past 12 to 18 generations of the Himba population.

"People may not be full first-cousins," Henn said. "But they may be half-cousins once removed and then their grandparents might have been half-cousins. Anytime something like that happens, it's going to contribute to there being identical DNA in the offspring."

A negative effect on fertility

The Himba are a pronatalist community, meaning they encourage their members to have many children. Typically, there are short intervals between births, roughly between one and three years, researchers said.

To gauge the effects of long ROH on fertility, the researchers measured the reproductive success of post-reproductive women in their sample population. The researchers defined reproductive success as the number of children who survived to at least 5.

The research team used statistical models to analyze the relationship between the amount of ROH in the genome and the number of children a woman had. They found that the greater the proportion of the genome that was in ROH, the more likely a woman was to have fewer children than a woman who had less ROH.

"This means that a woman who has parents who are more related is more likely to have fewer children throughout her lifetime than a woman who has parents who are less related," said Swinford.

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Entomologists issue warning about effects of climate change on insects

In a new scientific review, a team of 70 scientists from 19 countries warned that if no steps are taken to shield insects from the consequences of climate change, it will "drastically reduce our ability to build a sustainable future based on healthy, functional ecosystems."

Citing research from around the world, the team painted a bleak picture of the short- and long-term effects of climate change on insects, many of which have been in a state of decline for decades. Global warming and extreme weather events are already threatening some insects with extinction -- and it will only get worse if current trends continue, scientists say. Some insects will be forced to move to cooler climes to survive, while others will face impacts to their fertility, life cycle and interactions with other species.

Such drastic disruptions to ecosystems could ultimately come back to bite people, explained Anahí Espíndola, an assistant professor of entomology at the University of Maryland and one of the paper's co-authors.

"We need to realize, as humans, that we are one species out of millions of species, and there's no reason for us to assume that we're never going to go extinct," Espíndola said. "These changes to insects can affect our species in pretty drastic ways."

Insects play a central role in ecosystems by recycling nutrients and nourishing other organisms further up the food chain, including humans. In addition, much of the world's food supply depends on pollinators like bees and butterflies, and healthy ecosystems help keep the number of pests and disease-carrying insects in check.

These are just a few of the ecosystem services that could be compromised by climate change, the team of scientists cautioned. Unlike mammals, many insects are ectotherms, which means they are unable to regulate their own body temperature. Because they are so dependent on external conditions, they may respond to climate change more acutely than other animals.

One way that insects cope with climate change is by shifting their range, or permanently relocating to places with lower temperatures. According to one study cited by Espíndola and other scientists, the ranges of nearly half of all insect species will diminish by 50% or more if the planet heats up 3.2°C. If warming is limited to 1.5°C -- the goal of the global Paris Agreement on climate change -- the ranges of 6% of insects will be affected.

Espíndola, who studies the ways in which species respond to environmental changes over time, contributed to the sections of the paper that address range shifts. She explained that drastic changes to a species' range can jeopardize their genetic diversity, potentially hampering their ability to adapt and survive.

On the other hand, climate change may make some insects more pervasive -- to the detriment of human health and agriculture. Global warming is expected to expand the geographical range of some disease vectors (such as mosquitoes) and crop-eating pests.

"Many pests are actually pretty generalist, so that means they are able to feed on many different types of plants," Espíndola said. "And those are the insects that -- based on the data -- seem to be the least negatively affected by climate change."

The team noted that the effects of climate change are often compounded by other human-caused impacts, such as habitat loss, pollution and the introduction of invasive species. Combined, these stressors make it more difficult for insects to adapt to changes in their environment.

Though these effects are already being felt by insects, it is not too late to take action. The paper outlined steps that policymakers and the public can take to protect insects and their habitats. Scientists recommended "transformative action" in six areas: phasing out fossil fuels, curbing air pollutants, restoring and permanently protecting ecosystems, promoting mostly plant-based diets, moving towards a circular economy and stabilizing the global human population.

The paper's lead author, Jeffrey Harvey of the Netherlands Institute of Ecology (NIOO-KNAW) and Vrije Universiteit Amsterdam, said in a statement that urgent action is needed to protect insects and the ecosystems they support.

"Insects are tough little critters, and we should be relieved that there is still room to correct our mistakes," Harvey said. "We really need to enact policies to stabilize the global climate. In the meantime, at both government and individual levels, we can all pitch in and make urban and rural landscapes more insect-friendly."

The paper suggested ways that individuals can help, including managing public, private or urban gardens and other green spaces in a more ecologically-friendly way -- for instance, incorporating native plants into the mix and avoiding pesticides and significant changes in land usage when possible.

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