Table 1.

Mean and median distances of the two conserved markers of the protostomian break, AU contents of the whole 28S sequence and the region near the hidden break and proportion of sequences with the UAAU 4-mer per phylum.

	No. of seqs	Mean	Median	mean (AT) - whole 28S	mean(AT) - break region	% of species with UAAU
Protostomes with hidden break
Arthropoda	4,175	128.92	106	45.95%	51%	30.5%
Mollusca	555	107.43	105	42.79%	55.27%	7.2%
Platyhelminthes	147	117.30	116	49.72%	52.42%	23.12%
Nematoda (Enoplea)	18	141.22	118	50.50%	60.02%	22.2%
Nemertea	94	105.52	103	44.24%	53.86%	12.7%
Gastrotricha	74	101.32	102	48.23%	60.74%	75%
Halicryptus	1	95	95	53.66%	53.09%	N/A
Protostomes without hidden break
Nematoda (Chromadorea)	275	92.30	92	50.75%	53.56%	51.6%
Tardigrada	7	139	132	45.71%	45.64%	28.5%
Priapulus	1	89	89	55.10%	54.16%	N/A
Dicyema	1	78	78	43.35%	40.74%	N/A
Deuterostomes
Chordata	136	92.72	93	38.82%	33.98%	0%
Echinodermata	71	99.38	100	41.67%	29.94%	0%
Xenacoelomorpha	115	86.95	86	51.72%	52.29%	11.3%

In protostome phyla that possess the break, the distances between the two markers are in general larger than in the protostomes that have lost the break (with the exception of 2 species of tardigrades) and deuterostomes. The region that containing the break site has higher AU content compared to the average of the whole 28S molecule in break-possessing phyla, which is not the case in the species without the break. The UAAU tetranucleotide is not found in most protostome phyla with a hidden break and was not found in deuterostome phyla (except Xenacoelomorpha).