The following passage is adapted from Ricki Lewis, "Did Donkeys Arise from an Inverted Chromosome?", originally published 2018 in PLOSOne Blogs.

In the world of genome sequencing, donkeys haven’t received nearly as much attention as horses. But now a report on a new-and-improved genome sequence of Willy, a donkey (Equus asinus) jack 5 born at the Copenhagen Zoo in 1997, appears in the new issue of Science Advances, from Gabriel Renaud, of the Centre for GeoGenetics, Natural History Museum of Denmark. (A female is a jenny or jennet.) The new view provides clues to how donkeys may have branched from horses along the tree of evolution.

Horses and their relatives, past and present, are genetically peculiar in that their chromosomes are rearranged, with respect to each other. That should  prevent them from producing viable hybrids – yet they do. Donkeys have 62 chromosomes and horses have 64. A mule comes from the mating of a male donkey and a female horse, and has 63 chromosomes. Mules are known for their  intelligence, calm, stamina, and persistence. Their horse-like bodies perched on donkey-like limbs make them ideal for hauling tourists around the Grand Canyon and schlepping supplies in combat situations. The ears are large like those of the horse mom, and mules make a sound that begins as a whinny and becomes a bray.

The complementary couple, a female donkey and a male horse, produces a hinny, smaller than a mule. Hinnies are the flip side of the mule, with a donkey’s physique atop horsey limbs, and short donkey ears. They’re rarer than mules, but also have 63 chromosomes. It’s easy to mix them up.

Comparing Willy’s genome to a horse genome revealed their close evolutionary relationship. Only about 15% of horse genes aren’t also in the donkey genome, and only about 10% of a donkey’s genes don’t have counterparts in the horse. Most of the genes that they share provide basic “housekeeping” functions like dismantling proteins, repairing DNA, enabling embryonic development, and controlling cell division. So that’s why a copy of each genome can smush together to yield mules and hinnies.

A second form of information encoded in genomes, in addition to the A, C, T, G sequence, is the pattern of whether the two variants of individual genes are different (heterozygous) or the same (homozygous). Many contiguous homozygous genes form a “run of homozygosity” (ROH).

An ROH indicates a chromosome chunk, perhaps as long as millions of DNA bases, that’s the same from each of an individual’s parents, who in turn inherited it from a shared ancestor, like a grandparent that cousins share. The longer the ROH, the more recent the shared ancestor, because it takes time for mutations to accrue that would break the sameness of the sequence.

Scrutinizing ROHs can reveal recent inbreeding and domestication, help to reconstruct possible branching patterns of evolution, and, more practically, help ancestry companies assign the DNA in spit samples to geographic areas where people’s ancestors might have come from. The new study compared ROHs for the three zebra and three ass species, confirming that Willy’s most recent ancestors were Somali wild asses.

The researchers used Chicago HiRise assembly technology to up the quality of Willy’s genome sequence. “This new assembly allowed us to identify fine chromosomal rearrangements between the horse and the donkey that likely played an active role in their divergence and, ultimately, speciation,” they write.

The bigger pieces enabled them to zero in on DNA sequences where chromosomes contort, such as inversions (where a sequence flips) or translocations (where different chromosome types exchange parts). These events could have fueled the reproductive isolation of small populations that can expand into speciation.

If eventually sperm with one inverted chromosome fertilized eggs with the same inversion, animals would have been conceived in which both copies of the chromosome are inverted – and they’d be fertile with each other, but not with horses. Once a subpopulation with the inversion became established, further genetic changes would separate them further from the ancestral horse.

Passage 1 is adapted from Emma Hart Willard, "Improving Female Education." Originally published in 1819.

If the improvement of the American female character, and that alone, could be affected by public liberality, employed in giving better means of instruction; such improvement of one half of society, and that half, which barbarous and despotic nations have ever degraded, would of itself be an object, worthy of the most liberal government on earth; but if the female character be raised, it must inevitably raise that of the other sex; and thus does the plan proposed, offer, as the object of legislative bounty, to elevate the whole character of the community.

As evidence that this statement does not exaggerate the female influence in society, our sex  need but be considered, in the single relation of mothers. In this character, we have the charge of the whole mass of individuals, who are to compose the succeeding generation; during that period of youth, when the pliant mind takes any direction, to which it is steadily guided by a forming hand. How important a power is given by this charge! yet, little do too many of my sex know how, either to appreciate or improve it. Unprovided with the means of acquiring that knowledge, which flows liberally to the other sex- having our time of education devoted to frivolous acquirements, how should we understand the nature of the mind, so as to be aware of the importance of those early impressions, which we make upon the minds of our children? -or how should we be able to form enlarged and correct views, either of the character, to which we ought to mold them, or of the means most proper to form them aright?

Considered in this point of view, were the interests of male education alone to be consulted, that of females becomes of sufficient importance to engage the public attention. Would we rear the human plant to its perfection, we must first fertilize the soil which produces it. If it acquire its first bent and texture upon a barren plain, it will avail comparatively little, should it be afterwards transplanted to a garden.