Bringing Genetics To Xenopus: Half The Genome, Twice As Fast

The pipid frog Xenopus laevis has been among the most productive model systems for vertebrate experimental embryology, due to the large size, external development, robustness, and ease of molecular and surgical manipulation of its embryos.  One reason amphibian embryos are superb for embryological manipulations is that each cell contains its own power source- a yolk supply- enhancing its ability to withstand the rigors of transplantation or to continue to differentiate in an explant or as an individual cell in simple salt solutions.

Recent contributions from this system to axis determination, embryonic inductions, morphogenesis, and signal transduction studies have important implications for research on human disease 3, 4, 5,6.  In particular, evolving molecular techniques have helped identify a wide variety of activities through the addition of gene products to X. laevis assays, typically through mRNA injection into embryos or incubation of explants with protein preparations 7, 8, 9, 10, 11.

However, in order to determine whether a newly-identified activity is actually required for a given process, it is neccessary to subtract a given gene function.  In complex developmental systems, tools provided by genetics- for example, the phenotype of a null mutant- provide the strongest proof of participation.

Further impetus for developing an amphibian genetic model derives from a recent breakthrough which allows transgenic frogs to be produced cheaply, efficiently, and in large numbers (see What We Know So Far  1).  Transgenesis-related genetic approaches, combined with the frog's embryological advantages and the low cost of husbandry and animal housing, should permit the dissection of many basic developmental processes quickly and inexpensively relative to other vertebrate model systems.

However, Xenopus workers currently lack genetic tools for dissecting complex biological pathways.  X. laevis has two significant drawbacks as a genetic system.  First, it usually takes1-2 years to reach sexual maturity, reducing the practicality of many types of multi-generation experiments (e.g. making a line of frogs bearing reporter or marker constructs for transplantation experiments or breeding loci to homozygosity).

Second, it is allotetraploid, such that many genes are represented by extra copies which may or may not be functional. This greatly complicates creating mutants (since it may be necessary to inactivate four copies of a gene) and analyzing gene regulation (since it may be difficult to distinguish genomic clones of pseudogenes from those of active loci).

A frog which is better suited to genetic approaches is Xenopus (Silurana) tropicalisX. tropicalis is a close relative of X. laevis24 (viable hybrids between the two species have been reported25), and shares virtually all of X. laevis' advantages as an embryological system. In addition, it features a much shorter generation time (3-4 months), and a smaller diploid genome (twenty chromosomes, with about 1.7 x 109 base pairs, versus thirty-six chromosomes and 3.1 x 109 bp for X. laevis)26.  Adult X. tropicalis, at 4-5 cm, are considerably smaller than the 10cm X. laevis, and consequently can be housed more efficiently; eggs are also somewhat smaller (0.6-0.7 mm vs 1-1.3 for X. laevis), but still amenable to manipulation, and are more abundant (1000-3000 per spawning versus 300-1000)27.  The genome size and number of chromosomes compare favorably to those of mouse (forty chromosomes, 3 x 109 bp)28 and zebrafish (fifty chromosomes, 1.6 x 109 bp29).

The embryological techniques and molecular assays which have been described for X. laevis are readily applied to X. tropicalis, but may be supported by multigeneration genetic analyses.  X. tropicalis' diploid genome will facilitate uncovering recessive phenotypes, and pseudogenes are far less likely to complicate promoter analysis than in the tetraploid X. laevis.  Using mutants or transgenic animals in highly-developed tissue transplantation regimes will facilitate analysis of individual animals containing tissues of more than one genotype.  Such genetic mosaic analyses have been very useful in studies of Drosophila embryogenesis, but are technically challenging in extant vertebrate models.

Further information:

  1. Draft of a commentary which appeared in the July 1998 issue of Trends in Genetics by E. Amaya, M. Offield and R. Grainger (Vol. 14, No. 7, pp. ).
  2. Background and Significance section from NIH grant application expanding on points in this introduction.

 Species

X. laevis 

 X. tropicalis

ploidy 

allotetraploid

diploid

N

18 chromosomes

10 chromosomes

genome size

3.1 x 109 bp

1.7 x 109 bp

temp. optima

16-22o

25-30o

adult size

10 cm

4-5 cm

egg size

1-1.3 mm

0.7-0.8 mm

eggs/spawn

300-1000

1000-3000

generation time

1-2 years

<5 months

 

X. tropicalis
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