Evolution of Mammalian Gametes and Gonads and External Genitalia

In our studies some years ago we found huge differences in absolute and relative testis mass occurred across the species of old endemic Australian rodents (Breed & Taylor, 2000, J.Mammal.81:758-768), with adult male hopping mice, Notomys alexis, invariably having remarkably small testes. However mating with, and fertilization of, several females can nevertheless take place within a short period of time. This surprisingly high fertility in the light of the animal's minute testes is probably due to, in large part, the very rapid sperm transport that we have found occurs in this species (Bauer & Breed, 2008, J.Zool.274:349-356).

We have also found that N. alexis has very large penile spines. Co-evolving with this is a highly muscular vagina in the female with the occurrence of locking during copulation for a variable period of time. Since, in this species, females are far more aggressive than males the length of the lock may be determined by the female, hence we suggest that, in this species this highly divergent distal genitalia anatomy may have evolved so that cryptic female choice can occur.

Investigators

• Bill Breed
• Chris Leigh
• Eleanor Peirce
• Melissa Bauer
• Tasha Speight

We are currently attempting to determine whether the molecules involved in sperm - egg interaction at the time of fertilization in eutherian mammals are conserved in marsupials albeit that they diverged from eutherians well over 100 million year ago. We have shown that, in collaboration with an American group, one of the key proteins on the eutherian sperm surface thought to be involved in egg coat binding, galactosyltransferase, is also present in marsupial sperm (Braundmeier, Breed & Miller,2008, Reprod.Fert.Dev.20:402-407).

In addition, in collaboration with Prof. Richard Oko from Queen's University at Kingston, Ontario, we are investigating whether several of the other molecules involved in binding the sperm to the egg coat as well as activating the egg at fertilization in eutherian mammals are also present in marsupials with our preliminary observations so far suggesting that this is indeed the case. In our comparative studies on the evolution of egg coat glycoproteins in the native rodents we have found that, in at least a few lineages, the putative sperm combining region has undergone positive selection (Swann, Cooper & Breed,2007, Reproduction133:697-708). We are now in the process of extending this work to other clades of rodents to determine how widespread this phenomenon is.

Investigators

• Bill Breed
• Chris Swann
• Chris Leigh
• Steve Cooper

Rodents have remarkably variable sperm morphology across species and we are currently carrying out studies to determine the reasons for this. Our work has shown that a few species of Notomys invariably produce highly polymorphic sperm (Bauer & Breed,2006, Reprod.Fert.Dev.18:797-805; Breed et al.,2007, J.Zool.274:299-304), whereas in other species monomorphic, and far more complex, spermatozoa are produced where there are two elaborate cytoskeletal processes that extend from the upper concave surface of the sperm head (Breed, Idriss, Leigh & Oko,2009, Reprod.Fert.Dev.21:438-439).

Honours student, Dan Ding, working in our group, has clearly shown that the more complex sperm form is the ancestral type at least within Notomys and we believe that the simplification of sperm form that has evolved in a few clades is due to a relaxation of intermale sperm competition as a result of the evolution of a monogamous mating system.

Investigators

• Bill Breed
• Chris Leigh
• Mario Ricci
• Jeremy Austin

Our long term studies on the arid zone old endemic rodent, The Spinifex Hopping Mouse, Notomys alexis, in the south of the Northern Territory have shown suppression of female reproductive activity at high population density (Breed & Leigh,2011, Acta Zool.91:279-294) although males continue to produce sperm at these times, albeit of high variability (Bauer & Breed,2008, Reprod.Fert.Dev.18:799-805;). A second separate study carried out at Arid Recovery near Roxby Downs by honours student, Karleah Trengove, has in general supported these conclusions.

Other studies carried out on the environmental control of reproduction include an investigation into the effects of heat stress on sperm quality in hopping mice and house mice by PhD student, Harsha Wechalekar. Another PhD student, Liberty Olds, is studying population changes in small mammals in the Kimberley region of Western Australia and she appears to have discovered at least one new, as yet undescribed, native rodent species in the area in which she works.

A Thai student, Nuttawat Thitipramote, who has recently completed his PhD in our laboratory, worked on a species of rodent, The Greater Bandicoot Rat, Bandicota indica, in the rice fields in Thailand and showed that reproduction of this species may occur throughout the year although its extent during the dry season varied markedly and correlated with the amount of rain in the preceding wet season; his studies have also suggested a highly divergent breeding system and social organisation in this species from that of other coexisting rodents (Thitipramote et al., 2009, Current Zool. 55:48-55; Thitipramote et al, 2011, Acta Zool.91:201-205).

Investigators

• Bill Breed
• Brian Setchell
• David Taggart
• Chris Leigh
• Liberty Olds
• Karleah Trengove
• Harsha Wechalekar
• Nuttawat Thitipramote

Over the last few years cryopreservation of house mouse sperm has been successfully developed in several laboratories overseas. Using the Australian rodents, and in particular Pseudomys australis and Notomys alexis, we have managed to develop a superovulation regime for these species and, in collaboration with researchers at ReproMed, we are now determining the optimal conditions for the cryopreservation of their spermatozoa.

Investigators

• Bill Breed
• David Taggart
• Hassan Bakos
• Michelle Lane
• Nicole Palmer