Reproductive Isolation Barriers: The hurdles
of cross species families
By Nicholas
File: 16001b
Patients: Chip Maplewood and Gadget Hackwrench
==Introduction=
Mr Chip and Miss Gadget were referred to me by my colleagues at the Family and
Health department. Both appeared healthy and reported no medical conditions
and were members of the sentient strains of their respective species. Mr Chip
was a male Eastern chipmunk (Tamias striatus), in his mid 30s, while Miss Gadget
was a startling female house mouse (Mus musculus domesticus) in her mid 20s.
Thus I was taken aback when the couple, who were planning to marry, approached me and stated earnestly their desire to start a family. I explained that even though both were members of the order Rodentia, their plan would not only transgress species boundaries but also family boundaries. While species boundaries might be crossed from time to time, especially among closely related species such as the panthro (tigers, lions) species, a family boundary would be an impossible jump. It was then; Miss Gadget requested I do a karyotype analysis on both Mr Chip and her self. As of the test, it’s no more complicated then taking a photograph of stained chromosomes, and would require several hours to complete. Which I had assumed it would come to naught. Thus it was with this initial scepticism that I ordered the test carried out.
==Observations.==
Hybridisation under normal conditions is a rare event, as a number of isolation
mechanisms come into play. Whilst talking to Mr Chip and Miss Gadget, I discovered
how they had over some of these mechanisms.
Spatial isolation; for hybridisation to take place both parent species must be present in the same geographical location. It appears that both Mr Chip and Miss Gadget had worked closely together for many years now and importantly shared the same home, namely The Rescue Ranger Headquarters.
Ecological Isolation: Even in the same area, two populations can be genetically isolated by adaptation to living in different habitats or ecological niches. This seemed to be true from the most part as Miss Gadget spends most of her time in the workshop building/repairing equipment while Mr. Chip would be found in the hall reading the his latest novel or in the police station looking for cases. However such separation may be incomplete because the habitats may meet in certain areas within the ranges of both species, thus giving them the opportunity to cross. And again this was true as both Mr Chip and Miss Gadget meet daily at the breakfast table in the kitchen, over cups of coffee. Which I must observe, Miss Gadget enjoys as she emptied my office of its’ supply.
Ethological or behavioral isolation, occurs when members of the opposite sex of different species don't acknowledge each other because neither presents the correct mating rituals. How much of a barrier this has proved, isn’t certain. But it was noted in my conversation with Mr Chip, that despite numerous amorous advances on his part, Miss Gadget failed to respond or even pay notice. It was only when tragedy stuck and a crisis of impending death did she reconcile her feelings. Although not said, there were suggestions that Miss Gadget had come to yearn for children, tipping the balance towards their present situation. This in my opinion resulted from the near tragedies she had come to experience.
Temporal isolation; prevents fertilization because the two parent species reproduce at different times. This of course wasn’t a grave problem as house mice breed all year round. Chipmunks breed during the late winter, usually between February and April.
Mechanical isolation; deals with the actual mechanics of reproduction. However due to the matter’s delicate nature, I let the conversation skirt around the subject. Neither Mr Chip nor Miss Gadget was inclined to raise the topic. However, mechanical isolation is of trivial importance among mammalian species of similar size. It is of some significance among some insect species, though not all as many as one would expect.
Thus far, Mr Chip and Miss Gadget had circumnavigated all prezygotic barriers to starting a family. The artificial environment of modern living, and almost certainly the stress and constant danger of their occupation, compounded by Miss Gadget’s reclusive nature (something which struck me) had resulted in their odd choice of life partners. However the more important postzygotic barriers, barriers to the development of the child itself, cannot be overcome by any known means. Then the karyotype analysis returned.
A karyotype analysis, or karyotyping is a simple and importantly cheap method of examining chromosome structure. Variation in DNA compaction along a chromosome, results in the formation of bands when the chromosome is stained with a dye. These banding patterns are so distinct that it is possible and used to identify individual chromosomes.
Pulling out the photographs, I was astonished. Neither Miss Gadget’s nor Mr Chip’s chromosomes showed banding patterns appropriate for their respective species. Instead their patterns where similar to each other, appearing somewhat like a blend of chipmunk and mouse chromosomes. In addition several chromosome pairs in both Miss Gadget and Mr Chip had their centromeric structure reorganised, giving Mr Chip and Miss Gadget a chromosome profile that appeared identical! Together, this potentially allowed Miss Gadget and Mr Chip chromosomes to pair up. Chromosome paring ensure both daughter cells born in a process called mitosis inherit identical sets of chromosomes. But how could such a dramatic reorganisation can occur was beyond my imagination
Mr Chip then related one of their adventures regarding a human named Norton Nimnul and a device of his making called the Modemiser. The device, as Miss Gadget explained decomposed an object into energy before transmitting it down a telephone line. I was rather taken aback by her level of technical knowledge which she threw about like a child’s toy. Apparently a malfunction in the device resulted in the scrambling of body parts between Rangers. She repaired the device by substituting a burned out component with a paper clip.
This rather inappropriate ‘repair’ I suspect, is the cause for genomic scramble observed in the karyotype analysis. Only a full DNA sequencing can resolve how far has this blending has occurred, but I suspect it is near complete. It is fortunate, 95% of the mammalian genome is composed of meaningless junk DNA, DNA that do not code for a function. Otherwise the process would undoubtedly prove lethal.
I also speculate that this junk DNA lowered the probability of functional genes being over written during the combination process. Furthermore overwriting this junk provided space for new incoming gene sequences entered by the Modermiser. Thus obeying the law of mass conservation when the Rangers returned to their proper condition.
With permission, I then requested a full blood analysis on Mr Chip and Miss Gadget. Although I’ve only received the preliminary report, it did confirm the karyotyping conclusion and my suspicions. Both Mr Chip’s and Miss Gadget’s blood cells displayed surface proteins of both mice and chipmunks, including some which appeared unique. I can only guess that this is true for all their cells.
I advised the couple to reframe from donating blood, as their blood to put it crudely was no longer purely “mouse or chipmunk” and might prove antigenic; giving any would be recipient ‘blood poisoning’. A similar problem might arise if they were the recipients instead. And considering the dangers of their profession it would be advisable if they started banking a personal supply of blood in the hospital’s blood bank.
Furthermore this raises the possibility that the gametic isolation mechanism has been breached. One of the first steps of fertilisation requires the binding of recognition molecules on the egg by complementary molecules on the sperm from the same species. As their cells now display a mix of surface molecules from both species, the probability of successful fertilisation has markedly increased.
Here I was forced to answer Mr Chip’s question. I had to admit at this point a child might be possible, especially in view that both chipmunks and mice have the same number of chromosome (20 pairs) and both parents now have what appears to be chromosomes of similar structure. The child might even be able to have children of its own, as its chromosomes could pair up, unlike the mule whose none pairing chromosomes resulting in sterility.
Yet, such a child would inherit the growth and developmental programme of both chipmunk and mouse. How these programmes would interact is uncertain. In the best-case scenario the developmental programmes compliment each other with minor conflict, such as in the case of Tiger-Lion hybrids. In the worst case, the conflict would be lethal. Here Haldane’s rule might come into play.
Haldane’s rule: when sterility and inviability is displayed in a hybrid, it is often the heterozygotic sex that is worst affected.
In mammals the heterozygotic sex is male (XY). Thus I noted to the couple, should a child be born it would most likely be a girl (XX). The cause of the Haldane’s rule remains much debated.
At this point I mentioned to Mr Chip, that chipmunks, although not significantly so, were larger than mice. Their child might become too large for Miss Gadget to deliver safely. The pregnancy could be fraught with complications, and if so a caesarean is a distinct possibility. Furthermore the gestation period of a sentient mouse (200 days) is dissimilar to that of a chipmunk (290 days.). Miss Gadget would be able to carry the child for a period of undetermined length before problems occur. Labour suppressant drugs might be prescribed to prevent premature birth.
I could give no further advice to the couple, except with tongue in cheek, ‘give it a try,’ -which I did not offer due to ethical considerations.
==Conclusion==
Having seen Mr Chip and Miss Gadget from my office, I can only wonder at the
thing called love and the very bizarreness of the technology of our world. Should
they attempt at a family several things can occur, no children, a sickly child,
a ‘normal’ healthy child or a combination of the two last possibilities.
Concerning a healthy child, two other possibilities are open. The child may
be able to reproduce with either parent species or incredibly constitute the
start of a new species of Rodentia. I can only wait and see what surprises this
amazing couple have installed for our community.
Prepared By
Edward Greentea MVMS