question: Why can’t hybrid animals (e.g. mules, ligers) reproduce? I know they “don’t have sex cells, which are needed for it”, but why?
comment: Wrong number of chromosome mostly. Just looked on the internet and saw that a horse has 64 chromosomes and a donkey 62. Therefore a mule (Horse X Donkey) has 63 chromosome. Germinal cells have half of the genetic material, so for a donkey a germinal cell has 31 chromosome and for a horse it's 32. Well for a mule, half of 63 is 31.5 and it's not really viable. But like /u/Fatbus said, it's not always the case it's rare but hybrids can be fertile.
passages: Several years ago, I saw confined in the Zoological Gardens of London a female hybrid from an English dog and jackal, which even in this the first generation was so sterile that, as I was assured by her keeper, she did not fully exhibit her proper periods; but this case, from numerous instances have occurred of fertile hybrids from these two animals, was certainly exceptional.
   A hybrid has normal viability but is typically deficient in terms of reproduction or is sterile. This is demonstrated by the mule and in many other well known hybrids. In all of these cases sterility is due to the interaction between the genes of the two species involved; to chromosomal imbalances due to the different number of chromosomes in the parent species; or to nucleus-cytoplasmic interactions such as in the case of "Culex" described above.
   Interspecific hybrids are bred by mating individuals from two species, normally from within the same genus. The offspring display traits and characteristics of both parents, but are often sterile, preventing gene flow between the species. Sterility is often attributed to the different number of chromosomes between the two species. For example, donkeys have 62 chromosomes, horses have 64 chromosomes, and mules or hinnies have 63 chromosomes. Mules, hinnies, and other normally sterile interspecific hybrids cannot produce viable gametes, because differences in chromosome structure prevent appropriate pairing and segregation during meiosis, meiosis is disrupted, and viable sperm and eggs are not formed. However, fertility in female mules has been reported with a donkey as the father.
   BULLET::::- Second, not all hybrids exhibit heterosis (see outbreeding depression).
  
  An example of the ambiguous value judgements imposed on hybrids and hybrid vigor is the mule. While mules are almost always infertile, they are valued for a combination of hardiness and temperament that is different from either of their horse or donkey parents. While these qualities may make them "superior" for particular uses by humans, the infertility issue implies that these animals would most likely become extinct without the intervention of humans through animal husbandry, making them "inferior" in terms of natural selection.
  
  Section::::Genetic and epigenetic bases.
   Sterility in a non-polyploid hybrid is often a result of chromosome number; if parents are of differing chromosome pair number, the offspring will have an odd number of chromosomes, which leaves them unable to produce chromosomally-balanced gametes. While that is undesirable in a crop such as wheat, for which growing a crop that produces no seeds would be pointless, it is an attractive attribute in some fruits. Triploid bananas and watermelons are intentionally bred because they produce no seeds and are also parthenocarpic.
  
  Section::::In different taxa.:In humans.
   Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. Such hybrids are generally infertile, due to the two different sets of chromosomes being unable to pair up during meiosis. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. The importance of hybridization in creating new species of animals is unclear, although cases have been seen in many types of animals, with the gray tree frog being a particularly well-studied example.
   Hybrid sterility can be caused by different closely related species breeding and producing offspring, these animals are usually sterile due to different numbers of chromosomes from the two parents, causing an imbalance in the resulting offspring making it viable but not fertile, this is the case with the mule.
  
  Sterility can also be caused by selective breeding, where a selected trait is closely linked to genes involved in sex determination or fertility, for example goats breed to be polled (hornless), this results in a high number of intersex individuals among the offspring, which are typically sterile. 
   Their fusion products (hybrid cells) will be TK+ (due to the human gene) and HGPRT+ (due to the mouse gene) and will multiply on the HAT medium, while the man and mouse cells will fail to do so. Experiments with other selective media can be planned in a similar fashion.
  
  Section::::Characteristics of somatic hybridization and cybridization.
  
  BULLET::::1. Somatic cell fusion appears to be the only means through which two different parental genomes can be recombined among plants that cannot reproduce sexually (asexual or sterile).
   Section::::Fertility.
  
  The fertility of hybrid big cat females is well documented across a number of different hybrids. This is in accordance with Haldane's rule: in hybrids of animals whose sex is determined by sex chromosomes, if one sex is absent, rare or sterile, it is the heterogametic sex (the one with two different sex chromosomes e.g. X and Y).
   Hybrid, "Helianthus annuus dwarf2" does not contain the hormone gibberellin and does not display heliotropic behavior. Plants treated with an external application of the hormone display a temporary restoration of elongation growth patterns. This growth pattern diminished by 35% 7–14 days after final treatment.
  
  Hybrid male sterile and male fertile flowers that display heterogeneity have a low crossover of honeybee visitation. Sensory cues such as pollen odor, diameter of seed head, and height may influence pollinator visitation of pollinators that display constancy behavior patterns.
  
  Section::::Threats and diseases.
   Another possible cause of sterility among species in which presence or absence of one or the other sex chromosome determines gender, is if one sex chromosome of one species has recessive alleles interacting with autosomal alleles of the mating species. This could cause the heterogametic sex chromosome in the hybrid to be inviable or sterile, but homogametic sex chromosome will be fertile (Futuyma 1997). Consequently, in species where presence or absence of a Y chromosome determines gender, for instance, individuals carrying XY chromosomes (males) will be sterile and those carrying XX (females) will be fertile. This is closely related to Haldane's rule.
   Species boundaries in plants may be weaker than in animals, and cross species hybrids are often possible. A familiar example is peppermint, "Mentha" × "piperita", a sterile hybrid between "Mentha aquatica" and spearmint, "Mentha spicata". The many cultivated varieties of wheat are the result of multiple inter- and intra-specific crosses between wild species and their hybrids. Angiosperms with monoecious flowers often have self-incompatibility mechanisms that operate between the pollen and stigma so that the pollen either fails to reach the stigma or fails to germinate and produce male gametes. This is one of several methods used by plants to promote outcrossing. In many land plants the male and female gametes are produced by separate individuals. These species are said to be dioecious when referring to vascular plant sporophytes and dioicous when referring to bryophyte gametophytes.
   Species are reproductively isolated by strong barriers to hybridisation, which include morphological differences, differing times of fertility, mating behaviors and cues, and physiological rejection of sperm cells or the developing embryo. Some act before fertilization and others after it. Similar barriers exist in plants, with differences in flowering times, pollen vectors, inhibition of pollen tube growth, somatoplastic sterility, cytoplasmic-genic male sterility and the structure of the chromosomes. A few animal species and many plant species, however, are the result of hybrid speciation, including important crop plants such as wheat, where the number of chromosomes has been doubled.
   Other examples where hybridogenesis is at least one of modes of reproduction include i.e.
  
  BULLET::::- Iberian minnow "Tropidophoxinellus alburnoides" ("Squalius pyrenaicus" × hypothetical ancestor related with Anaecypris hispanica)
  
  BULLET::::- spined loaches "Cobitis hankugensis" × "C. longicorpus"
  
  BULLET::::- "Bacillus" stick insects B. rossius × Bacillus grandii benazzii
  
  Section::::See also.
  
  BULLET::::- Charles Bonnet – conducted experiments that established what is now termed parthenogenesis in aphids
  
  BULLET::::- Jan Dzierżon - Polish apiarist and a pioneer of parthenogenesis among bees
  
  BULLET::::- Jacques Loeb – caused the eggs of sea urchins to begin embryonic development without sperm
  
  BULLET::::- Miraculous births
   Genetics are more variable and malleable in plants than in animals, probably reflecting the higher activity level in animals. Hybrids genetics will necessarily be less stable than those of species evolving through isolation, which explains why hybrid species appear more common in plants than in animals. Many agricultural crops are hybrids with double or even triple chromosome sets. Having multiple sets of chromosomes is called polyploidy (or polyploidity). Polyploidy is usually fatal in animals where extra chromosome sets upset fetal development, but is often found in plants. A form of hybrid speciation that is relatively common in plants, occurs when an infertile hybrid becomes fertile after doubling of the chromosome number.
   Plant hybrids often suffer from an autoimmune syndrome known as hybrid necrosis. In the hybrids, specific gene products contributed by one of the parents may be inappropriately recognized as foreign and pathogenic, and thus trigger pervasive cell death throughout the plant. In at least one case, a pathogen receptor, encoded by the most variable gene family in plants, was identified as being responsible for hybrid necrosis.
  
  Section::::Genetics.:Chromosomal rearrangements in yeast.
   Doubled haploids can be produced "in vivo" or "in vitro". Haploid embryos are produced "in vivo" by parthenogenesis, pseudogamy, or chromosome elimination after wide crossing. The haploid embryo is rescued, cultured, and chromosome-doubling produces doubled haploids. The "in vitro" methods include gynogenesis (ovary and flower culture) and androgenesis (anther and microspore culture). Androgenesis is the preferred method. Another method of producing the haploids is wide crossing. In barley, haploids can be produced by wide crossing with the related species "Hordeum bulbosum"; fertilization is affected, but during the early stages of seed development the "H. bulbosum" chromosomes are eliminated leaving a haploid embryo. In tobacco ("Nicotiana tabacum"), wide crossing with "Nicotiana africana" is widely used. When "N. africana" is used to pollinate "N. tabacum", 0.25 to 1.42 percent of the progeny survive and can readily be identified as either F1 hybrids or maternal haploids. Although these percentages appear small, the vast yield of tiny seeds and the early death of most seedlings provide significant numbers of viable hybrids and haploids in relatively small soil containers. This method of interspecific pollination serves as a practical way of producing seed-derived haploids of "N. tabacum", either as an alternative method or complementary method to anther culture.
   In many species where pre-copulatory reproductive isolation does not exist, hybrids are produced but they are of only one sex. This is the case for the hybridization between females of "Drosophila simulans" and "Drosophila melanogaster" males: the hybridized females die early in their development so that only males are seen among the offspring. However, populations of "D. simulans" have been recorded with genes that permit the development of adult hybrid females, that is, the viability of the females is "rescued". It is assumed that the normal activity of these speciation genes is to "inhibit" the expression of the genes that allow the growth of the hybrid. There will also be regulator genes.
   Section::::Similar phenomena.:Hybridogenesis.
  
  Hybridogenesis is a mode of reproduction of hybrids. Hybridogenetic hybrids (for example AB genome), usually females, during gametogenesis exclude one of parental genomes (A) and produce gametes with unrecombined genome of second parental species (B), instead of containing mixed recombined parental genomes. First genome (A) is restored by fertilization of these gametes with gametes from the first species (AA, sexual host, usually male).
  
  So hybridogenesis is not completely asexual, but instead hemiclonal: half of genome is passed to the next generation clonally, unrecombined, intact (B), other half sexually, recombined (A).
   Plant hybridization is more commonplace compared to animal hybridization. Many crop species are hybrids, including notably the polyploid wheats: some have four sets of chromosomes (tetraploid) or six (hexaploid), while other wheat species have (like most eukaryotic organisms) two sets (diploid), so hybridization events likely involved the doubling of chromosome sets, causing immediate genetic isolation.
   Post-copulatory isolation can also arise between chromosomally differentiated populations due to chromosomal translocations and inversions. If, for example, a reciprocal translocation is fixed in a population, the hybrid produced between this population and one that does not carry the translocation will not have a complete meiosis. This will result in the production of unequal gametes containing unequal numbers of chromosomes with a reduced fertility. In certain cases, complete translocations exist that involve more than two chromosomes, so that the meiosis of the hybrids is irregular and their fertility is zero or nearly zero. Inversions can also give rise to abnormal gametes in heterozygous individuals but this effect has little importance compared to translocations. An example of chromosomal changes causing sterility in hybrids comes from the study of "Drosophila nasuta" and "D. albomicans" which are twin species from the Indo-Pacific region. There is no sexual isolation between them and the F1 hybrid is fertile. However, the F2 hybrids are relatively infertile and leave few descendants which have a skewed ratio of the sexes. The reason is that the X chromosome of "albomicans" is translocated and linked to an autosome which causes abnormal meiosis in hybrids. Robertsonian translocations are variations in the numbers of chromosomes that arise from either: the fusion of two acrocentric chromosomes into a single chromosome with two arms, causing a reduction in the haploid number, or conversely; or the fission of one chromosome into two acrocentric chromosomes, in this case increasing the haploid number. The hybrids of two populations with differing numbers of chromosomes can experience a certain loss of fertility, and therefore a poor adaptation, because of irregular meiosis.
   Section::::Applications of DHs plant breeding.:Elite crossing.
  
  Traditional breeding methods are slow and take 10–15 years for cultivar development. Another disadvantage is inefficiency of selection in early generations because of heterozygosity.
  
  These two disadvantages can be over come by DHs, and more elite crosses can be evaluated and selected within less time.
  
  Section::::Applications of DHs plant breeding.:Cultivar development.
   Section::::Fertility.
  
  Guggisberg wrote that ligers and tigons were long thought to be sterile; in 1943, however, a 15-year-old hybrid between a lion and an "Island" tiger was successfully mated with a lion at the Munich Hellabrunn Zoo. The female cub, although of delicate health, was raised to adulthood.
   Plant species hybridize more readily than animal species, and the resulting hybrids are fertile more often. Many plant species are the result of hybridization, combined with polyploidy, which duplicates the chromosomes. Chromosome duplication allows orderly meiosis and so viable seed can be produced.
   A number of these genes have been found in the "melanogaster" species group. The first to be discovered was "Lhr" (Lethal hybrid rescue) located in Chromosome II of "D. simulans". This dominant allele allows the development of hybrid females from the cross between "simulans" females and "melanogaster" males. A different gene, also located on Chromosome II of "D. simulans" is "Shfr" that also allows the development of female hybrids, its activity being dependent on the temperature at which development occurs. Other similar genes have been located in distinct populations of species of this group. In short, only a few genes are needed for an effective post copulatory isolation barrier mediated through the non-viability of the hybrids.
  
presuppositions: Hybrid animals are never fertile.
corrections: Hybrid animals can be fertile, however fertility is rare.
raw_presuppositions: Hybrid animals are never fertile. Hybrids are never fertile.
raw_corrections: Hybrid animals can be fertile, however fertility is rare. Hybrids can rarely be fertile.