Anapsid v Diapsid II

Anapsid v Diapsid II

It appears the readership for this series is slightly larger than I originally thought!

Part I provided an overview of the problem of where Testudines fit within the Reptilia, and how this affects a traditional Anapsida. Part II will summarise research supporting various positions, namely:
• Anapsid hypothesis (AH) traditional morphological position
• Lepidosauria hypothesis (LH) Testudines are more closely related to Sphenodon and Squamata, leading to a paraphyletic Reptilia.
• Archosauria hypothesis (ArH) Testudines are more closely related to crocodiles and birds, again causing a paraphyletic Reptilia.
Most morphological work (but not all) supports the AH, I won’t summarise these, Part I lists some of the notable exceptions.

Molecular
With the advent of DNA sequencing a while new way to explore phylogenetic relationships was opened up. Early work produced differing results. α-enolase grouped Archosauria and Lepidosauria as clades with Testudines between the two (therefore paraphyletic) (Hedges 1994). α-globin chains supported a turtle:tuatara:avian clade, and the β-globin placed Testudines as an outgroup to a tuatara:avian clade that led to the Squamata. Hb (haemoglobin) chains (Δ and β’, β’’) placed tuatara closer to aves and turtles with crocodiles as a sistergroup. α-D globin genes demonstrated that turtles were a sistergroup to a tuatara:avian branch (Gorr et al. 1998).
Analysis of Reptilian sperm demonstrated high similarities between tuatara, turtles, crocodiles and to a lesser degree non-passerine aves. Sperm morphology clearly demonstrated the archaic relationships between these Orders, compared to the derived nature of Squamate sperm. mtDNA analysis rejected the AH and most strongly supported the ArH, although the LH couldn’t be statistically rejected (Healy and Jamieson 1992; Jamieson and Healy 1992; Healy and Jamieson 1994).

Reviewing all molecular data in 1999 Testudines and Crocodylia were found to group closely, excluding the Aves. Increasing the number of mtDNA basepairs for analysis grouped Testudines within the Archosaurian branch (ArH). Although recent analysis of full mtDNA from a large number of Reptilia placed Testudines as a Diapsid sistergroup to Archosauria and that clade as a sistergroup to the Lepidosauria (Rest et al. 2003).

As an interesting morphological comparison, a study on snake eye formation placed Crocodylia as basal to the Testudines and those two as basal to a Sphenodon:Squamata:aves clade (Caprette et al. 2004). Again a paraphyletic Reptilia.

Chromosomes
My interests involve chromosomal variation present within the Reptilia. Sphenodon show no variation in all populations sampled (Stephens Island, North Brother Island, Stanley Island, Poor Knights Island; Ruamahua-iti reflecting S. punctatus northern-North Island and Cook Strait groups, and S. guntheri) (Norris et al. 2004). Crocodylians show three chromosomal groupings based on chromosomal morphology (Bickham 1984), a situation that doesn’t reflect the two groupings seen in mtDNA analysis (Janke et al. 2005). I’ll come back to it later, but that point could be quite important. The level of chromosomal variation appears to correlate with morphology, and definitely with speciation.

Testudines are by far the most speciose of the archaic reptile lineages, numbering over 285 species (Zug et al. 2001). Chromosomal variaition is low; major macrochromosome variation is limited with most variation occurring in microchromosome number. It is reasonably easy to derive most Testudines from a primitive karyotype. Two families show a highly derived karyotype, Carettochelyidae and Trionychidae. Both families also who highly derived morphology from the Testudine lineage. The Aves also show quite limited chromosomal variation, generally a macrochromosomes complement of 2n=2x=10-20, with highly variable numbers of microchromosomes.

Squamata demonstrate high chromosome variation. Actually ‘high’ doesn’t even begin to cover it. Although 2n=36 is common, chromosome morphology is highly varied, making extrapolation of a primitive karyotype very difficult.

In one of those eureka moments, while sipping coffee with a mild hangover and idly flicking through a review of Reptilian karyology I noticed a number of tuatara chromosomes. Mislabelled as Testudines. Yeah, Testudine karyotypes are very very similar to tuatara. In fact the three archaic lineages show some similarities. Supporting an archaic chromosomal relationship between Sphenodon and Testudines. See my paper for more detail :)

Various work on genomic DNA sequences involved in sex determining genes resulted in various phylogenies. AMH (anti mullerian hormone) showed no variation between various tuatara populations and grouped with turtles, both splitting the Archosauria, with alligators grouping closer to mammals. WT1 (Wilms’ tumour 1, a sex determining and nephrology gene) split the northern and Cook Strait tuatara. Northern NI tuatara were closer to alligators, with turtles splitting the Archosauria. DMRT1 show tuatara to be highly derived. Testudines are basal to all reptiles and mammals, and the Squamata cause the Archosauria to become paraphyletic.

So where does this leave us? I think from a molecular perspective that there is very weak support for the AH. There is significantly more support for Testudines as members of the Diapsida, although where in the Diapsida they fit is a problem. Chromosomes and sperm suggest closer to Sphenodon (and therefore with the Lepidosauria), mtDNA, or rather the most recent mtDNA study using a larger dataset supported the ArH. gDNA showed various phylogenetic positions: AMH suggested turtles were closer to crocodiles (ArH); WT1 also supported the ArH, and DMRT1 supported a traditional LH as turtles appeared basal to all other animals in the analysis.

Strangely I have opinions on these too, but I’m going to save them for Pt III which is likely to explore my views on:
- reptilia relationships
- reptilian evolution
- anapsids, are there any?

As usual, any question/comments welcome !!

Love, B.

Bickham, J. W. (1984). Patterns and modes of chromosomal evolution in reptiles. Chromosomes in evolution of eukaryotic groups. A. K. Sharma and A. Sharma. Florida, USA, CRC Press. II: 13-40.
Caprette, C. L., M. S. Y. Lee, R. Shine, A. Mokany and J. F. Downhower (2004). "The origin of snakes (Serpentes) as seen through eye evolution." Biological Journal of the Linnean Society 81: 469-482.
Gorr, T. A., B. K. Mable and T. Kleinschmidt (1998). "Phylogenetic analysis of reptilian hemoglobins: Trees, rates, and divergence." Journal of Molecular Evolution 47: 471-485.
Healy, J. M. and B. G. M. Jamieson (1992). "Ultrastructure of the spermatozoon of the tuatara (Sphenodon punctatus) and its relevance to the relationships of the Sphenodontida." Philosophical Transactions of the Royal Society London B 335: 193-205.
Healy, J. M. and B. G. M. Jamieson (1994). "The ultrastructure of spermatogenesis and epididymal spermatozoa of the tuatara Sphenodon punctatus (Sphenodontida, Amniota)." Philosophical Transactions of the Royal Society London B 344: 187-199.
Hedges, S. B. (1994). "Molecular evidence for the origin of birds." Proceedings of the National Academy of Science 91: 2621-2624.
Jamieson, R. G. M. and J. M. Healy (1992). "The phylogenetic position of the tuatara, Sphenodon (Sphenodontida, Amniota), as indicated by cladistic analysis of the ultrastructure of spermatozoa." Philosophical Transactions Royal Society London B 335: 207-219.
Janke, A., A. Gullberg, S. Hughes, R. K. Aggarwal and U. Arnason (2005). "Mitogenomic analyses place the Gharial (Gavialis gangeticus) on the crocodile tree and provide pre-K/T divergence times for most Crocodilians." Journal of Molecular Evolution 61: 620-626.
Norris, T. B., G. K. Rickards and C. H. Daugherty (2004). "Chromosomes of tuatara, Sphenodon, a chromosome heteromorphism and an archaic reptilian karyotype." Cytogenetic and Genome Research 105(1): 93-99.
Rest, J. S., J. C. Ast, C. C. Austin, P. J. Waddell, E. A. Tibbetts, J. M. Hay and D. P. Mindell (2003). "Molecular systematics of primary reptilian lineages and the tuatara mitochondrial genome." Molecular Phylogenetics and Evolution 29: 289-297.
Zug, G. R., L. J. Vitt and J. P. Caldwell (2001). Herpetology: An introductory biology of amphibians and reptiles, Academic Press.