<p>Background: Robertsonian (Rb) chromosomal rearrangements are very common in mammals and are the primary basis of chromosome number variation between species. The fertility of heterozygotes has particular significance in understanding the mode of fixation of Rb rearrangements, and could have a role in the attainment of reproductive isolation by chromosomally differentiated species. Summary: Here we survey available data on fertility of Rb heterozygotes in mammals, comparing with homozygotes, and considering effects on litter size, frequencies of anaphase I nondisjunction, germ cell death and pachytene features associated with that germ cell death. We consider both simple heterozygotes which form trivalent configurations at meiosis I and complex heterozygotes which form longer configurations due to heterozygosity for different chromosomes with monobrachial homology. Two species have a particularly wide variety of Rb heterozygotes and have been well studied: the house mouse (the western subspecies) and the common shrew. The overall data confirm that heterozygosity for a single Rb metacentric may be associated with near-normal fertility in mammals, though not in every instance. Usually infertility is not going to be a substantial hindrance to fixation of Rb fusions or fissions. Nor is infertility in simple heterozygotes for one or a few Rb metacentrics on its own likely to promote reproductive isolation. However, simple heterozygotes forming many meiotic trivalents and complex heterozygotes forming long meiotic configurations may suffer substantial infertility or sterility. Even so, heterozygous house mice and common shrews forming the very longest meiotic chains and rings may produce some young. We discuss the implications of these findings with regards the role of Rb rearrangements in speciation. Key Messages: Infertility due to Rb heterozygosity on its own may rarely hinder fixation of Rb rearrangements nor be sufficient to cause a complete interruption to gene flow between hybridizing chromosomal forms. However, this does not rule out a role for Rb rearrangements in speciation. Reinforcement is possible, and Rb rearrangements have the potential to act in synergy with genic incompatibilities to promote reproductive isolation. There can also be the contrary process of despeciation. Natural selection may respond in various ways to a given degree of infertility. </p>.

In the present study, 10 allotriploid (3nALT) and 10 allopentaploid (5nALP) six-month-old hybrid fish and two 3nALT and four 5nALP 40-month-old hybrid fish, which resulted by crossing female Russian sturgeon <i>Acipenser gueldenstaedtii</i> (Brandt and Ratzeberg, 1833) and male American paddlefish <i>Polyodon spathula</i> (Walbaum, 1792), were investigated. It was revealed that six-month-old 3nALT and 5nALP hybrids initially had "undifferentiated" gonads, while in the 40-month-old hybrids, only testes were observed in one case of 3nALT and one case of 5nALP hybrids. The testis of 3nALT hybrids was partially developed with spermatogonia, while the testis of one 5nALP hybrid was in the second developmental stage with low spermatogonia density. We could not determine gonad differentiation in any of the cases when the hybrid individuals had the W sex chromosome. We concluded that the gonad differentiation of these interfamilial hybrids follows a similar pattern to interspecific hybrids of different ploidy parent species of the family <i>Acipenseridae</i>, which is consistent with the classical Haldane's rule. However, it cannot be excluded that the testis of this/these hybrid(s) may produce fertile sperm after sexual maturity, depending on additional genetic, hormonal and environmental factors, and further research is required for its evaluation.

Chimpanzees and humans are closely related. Genetic animal hybrids with different chromosome numbers decrease the probability of viable offspring and rarely The humanzee (sometimes chuman, manpanzee or chumanzee) is a hypothetical hybrid of chimpanzee and human, thus a form of human-nonhuman hybrid. Serious attempts to create such a hybrid were made by Soviet biologist Ilya Ivanovich Ivanov in the 1920s, and possibly by researchers in China in the 1960s; however, neither succeeded. The possibility of hybrids between humans and other apes has been entertained since at least the medieval period; Saint Peter Damian (11th century) claimed to have been told of the offspring of a human woman who had mated with a non-human ape, and so did Antonio Zucchelli, an Italian Franciscan capuchin friar who was a missionary in Africa from 1698 to 1702, and Sir Edward Coke in "The Institutes of the Lawes of England". Chimpanzees and humans are closely related. Genetic animal hybrids with different chromosome numbers decrease the probability of viable offspring and rarely occur in the first cross. Evolutionary biologists have found evidence that hybridization between humans and Pan troglodytes resulted in some varieties of archaic humans. Chimpanzees and bonobos are separate species, but hybridization has been documented. Genetic similarity, and thus the chances of successful hybridization, is not always correlated with visual appearances. Domestication and back-crossing has been found to increase fertility in subsequent generations. All great apes have similar genetic chromosome structure. Humans have one pair fewer chromosomes than other apes; humans have 23 chromosome pairs, while all other apes have 24, with ape chromosomes 12 and 13 fused in the human genome into a large chromosome (which contains remnants of the centromere and telomeres of the ancestral 12 and 13). Chromosomes 6, 13, 19, 21, 22, and X are structurally the same in all great apes. Chromosomes 3, 11, 14, 15, 18, and 20 match among gorillas, chimpanzees, and humans. Chimpanzees and humans match on 1, 2p, 2q, 5, 7–10, 12, 16, and Y as well. Some older references include Y as a match among gorillas, chimpanzees, and humans, but chimpanzees, bonobos, and humans have recently been found to share a large transposition from chromosome 1 to Y not found in other apes. The degree of chromosomal similarity among apes is roughly equivalent to that found in equines. Interfertility of horses and donkeys is common, although sterility of the offspring (mules) is more common. Complexities and partial sterility pertain to horse–zebra hybrids, or zorses, whose chromosomal disparity is very wide, with…